Display device

ABSTRACT

A protective circuit includes a non-linear element, which further includes a gate electrode, a gate insulating layer covering the gate electrode, a pair of first and second wiring layers whose end portions overlap with the gate electrode over the gate insulating layer and in which a conductive layer and a second oxide semiconductor layer are stacked, and a first oxide semiconductor layer which overlaps with at least the gate electrode and which is in contact with side face portions of the gate insulating layer and the conductive layer of the first wiring layer and the second wiring layer and a side face portion and a top face portion of the second oxide semiconductor layer. Over the gate insulating layer, oxide semiconductor layers with different properties are bonded to each other, whereby stable operation can be performed as compared with Schottky junction. Thus, the junction leakage can be decreased and the characteristics of the non-linear element can be improved.

TECHNICAL FIELD

The present invention relates to a display device including an oxide semiconductor.

BACKGROUND ART

A thin film transistor formed over a flat plate such as a glass substrate is manufactured using amorphous silicon or polycrystalline silicon, as typically seen in a liquid crystal display device. A thin film transistor manufactured using amorphous silicon has low field effect mobility, but such a transistor can be formed over a glass substrate with a larger area. On the other hand, a thin film transistor manufactured using polycrystalline silicon has high field effect mobility, but a crystallization step such as laser annealing is necessary and such a transistor is not always suitable for a larger glass substrate.

In view of the foregoing, attention has been drawn to a technique by which a thin film transistor is manufactured using an oxide semiconductor, and such a transistor is applied to an electronic device or an optical device. For example, Patent Document 1 and Patent Document 2 disclose a technique by which a thin film transistor is manufactured using zinc oxide (ZnO) or an In—Ga—Zn—O based oxide semiconductor as an oxide semiconductor film and such a transistor is used as a switching element or the like of an image display device.

-   [Patent Document 1] Japanese Published Paten Application No.     2007-123861 -   [Patent Document 2] Japanese Published Paten Application No.     2007-96055

DISCLOSURE OF INVENTION

A thin film transistor in which a channel formation region is formed using an oxide semiconductor has characteristics as follows: the operation speed is higher than that of a thin film transistor including amorphous silicon and the manufacturing process is simpler than that of a thin film transistor including polycrystalline silicon. That is, the use of an oxide semiconductor makes it possible to manufacture a thin film transistor with high field effect mobility even at low temperatures of 300° C. or lower.

In order to take advantage of the features of a display device including an oxide semiconductor, which is superior in operating characteristics and capable of manufacture at low temperatures, a protective circuit and the like including appropriate structures are necessary. Moreover, it is important to ensure the reliability of the display device including an oxide semiconductor.

An object of an embodiment of the present invention is to provide a structure which is suitable as a protective circuit.

In a display device intended for a variety of purposes manufactured by stacking, in addition to an oxide semiconductor, an insulating film and a conductive film, an object of an embodiment of the present invention is to enhance the function of a protective circuit and stabilize the operation.

An embodiment of the present invention is a display device in which a protective circuit is formed using a non-linear element including an oxide semiconductor. This non-linear element includes a combination of oxide semiconductors with different oxygen contents.

An illustrative embodiment of the present invention is a display device which includes scan lines and signal lines provided over a substrate having an insulating surface so as to intersect with each other, a pixel portion in which pixel electrodes are arranged in matrix, and a non-linear element formed from an oxide semiconductor in a region outside the pixel portion. The pixel portion includes a thin film transistor in which a channel formation region is formed in a first oxide semiconductor layer. The thin film transistor in the pixel portion includes a gate electrode connected to the scan line, a first wiring layer which is connected to the signal line and which is in contact with the first oxide semiconductor layer, and a second wiring layer which is connected to the pixel electrode and which is in contact with the first oxide semiconductor layer. Moreover, the non-linear element is provided between the pixel portion and a signal input terminal disposed at the periphery of the substrate. The non-linear element includes a gate electrode and a gate insulating layer covering the gate electrode; a pair of a first wiring layer and a second wiring layer which is formed by stacking a conductive layer and a second oxide semiconductor layer and whose end portion overlaps with the gate electrode over the gate insulating layer; and a first oxide semiconductor layer which overlaps with at least the gate electrode and which is in contact with side face portions of the gate insulating layer and the conductive layer in the first wiring layer and the second wiring layer and a side face portion and a top face portion of the second oxide semiconductor layer. The gate electrode of the non-linear element is connected to the scan line or the signal line and the first wiring layer or the second wiring layer of the non-linear element is connected to the gate electrode via a third wiring layer so that the potential of the gate electrode is applied to the first wiring layer or the second wiring layer.

An illustrative embodiment of the present invention is a display device which includes scan lines and signal lines provided over a substrate having an insulating surface so as to intersect with each other, a pixel portion including pixel electrodes arranged in matrix, and a protective circuit in a region outside the pixel portion. The pixel portion includes a thin film transistor in which a channel formation region is formed in a first oxide semiconductor. The thin film transistor in the pixel portion includes a gate electrode connected to the scan line, a first wiring layer which is connected to the signal line and which is in contact with the first oxide semiconductor layer, and a second wiring layer which is connected to the pixel electrode and which is in contact with the first oxide semiconductor layer. In the region outside the pixel portion, a protective circuit for connecting the scan line and a common wiring to each other and a protective circuit for connecting the signal line and a common wiring to each other are provided. The protective circuit includes a gate electrode; a gate insulating layer covering the gate electrode; a pair of a first wiring layer and a second wiring layer which is formed by stacking a conductive layer and a second oxide semiconductor layer and whose end portion overlaps with the gate electrode over the gate insulating layer; and a first oxide semiconductor layer which overlaps with at least the gate electrode and which is in contact with side face portions of the gate insulating layer and the conductive layer in the first wiring layer and the second wiring layer and a side face portion and a top face portion of the second oxide semiconductor layer. Moreover, the gate electrode of the non-linear element is connected to the first wiring layer or the second wiring layer via a third wiring layer.

Here, the first oxide semiconductor layer includes oxygen at higher concentration than the second oxide semiconductor layer. That is, the first oxide semiconductor layer is oxygen-excess type, while the second oxide semiconductor layer is oxygen-deficiency type. The first oxide semiconductor layer has lower electrical conductivity than the second oxide semiconductor layer. The first oxide semiconductor layer has an amorphous structure, and the second oxide semiconductor layer includes a nanocrystal in an amorphous structure in some cases.

Note that the ordinal numbers such as “first” and “second” in this specification are used for convenience and do not denote the order of steps and the stacking order of layers. In addition, the ordinal numbers in this specification do not denote particular names which specify the invention.

In this specification, a semiconductor film formed from an oxide semiconductor including In, Ga, and Zn is also referred to as “an IGZO semiconductor film” and a semiconductor layer formed from such an oxide semiconductor is also referred to as “an IGZO semiconductor layer.”

According to an embodiment of the present invention, a display device having a structure suitable as a protective circuit can be provided by forming the protective circuit with use of a non-linear element including an oxide semiconductor. In the connection structure between the first oxide semiconductor layer of the non-linear element and the wiring layers, the provision of the region which is bonded with the second oxide semiconductor layer, which has higher electrical conductivity than the first oxide semiconductor layer, allows stable operation as compared with the case of using only metal wirings. Accordingly, the function of the protective circuit is enhanced and the operation can be made stable.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a positional relationship among signal input terminals, scan lines, signal lines, protective circuits including non-linear elements, and a pixel portion in a display device.

FIG. 2 illustrates an example of a protective circuit.

FIG. 3 illustrates an example of a protective circuit.

FIGS. 4A and 4B are plan views illustrating an example of a protective circuit.

FIG. 5 is a cross-sectional view illustrating an example of a protective circuit.

FIGS. 6A to 6C are cross-sectional views illustrating a process for manufacturing a protective circuit.

FIGS. 7A to 7C are cross-sectional views illustrating a process for manufacturing a protective circuit.

FIGS. 8A to 8C are cross-sectional views illustrating a process for manufacturing a protective circuit.

FIGS. 9A to 9C are cross-sectional views illustrating a process for manufacturing a protective circuit.

FIG. 10 is a cross-sectional view of electronic paper.

FIGS. 11A and 11B are each a block diagram of a semiconductor device.

FIG. 12 illustrates a structure of a signal line driver circuit.

FIG. 13 is a timing chart of operation of a signal line driver circuit.

FIG. 14 is a timing chart of operation of a signal line driver circuit.

FIG. 15 is a diagram illustrating a structure of a shift register.

FIG. 16 illustrates a connection structure of a flip-flop of FIG. 14.

FIGS. 17A and 17B are top views and FIG. 17C is a cross-sectional view, each illustrating a semiconductor device of Embodiment 6.

FIG. 18 is a cross-sectional view illustrating a semiconductor device of Embodiment 6.

FIG. 19 illustrates an equivalent circuit of a pixel in a semiconductor device of Embodiment 7.

FIGS. 20A to 20C each illustrate a semiconductor device of Embodiment 7.

FIG. 21A is a top view and FIG. 21B is a cross-sectional view, both describing a semiconductor device of Embodiment 7.

FIGS. 22A and 22B illustrate examples of applications of electronic paper.

FIG. 23 is an external view illustrating an example of an electronic book device.

FIG. 24A is an external view of an example of a television device and FIG. 24B is an external view of an example of a digital photo frame.

FIGS. 25A and 25B are external views illustrating examples of game machines.

FIG. 26 is an external view illustrating an example of a cellular phone.

FIGS. 27A and 27B are plan views illustrating an example of a protective circuit.

FIGS. 28A and 28B are plan views illustrating an example of a protective circuit.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention are described below with reference to the drawings. The present invention is not limited to the description below and it is easily understood by those skilled in the art that the mode and details can be changed variously without departing from the scope and spirit of the present invention. Therefore, the present invention should not be interpreted as being limited to what is described in the embodiments described below. Note that a reference numeral denoting the same portion in all figures is used in common in the structures of the present invention which is explained below.

Embodiment 1

In Embodiment 1, an example of a display device including a pixel portion and a protective circuit including a non-linear element provided around the pixel portion is described with reference to drawings.

FIG. 1 illustrates a positional relationship signal input terminals, scan lines, signal lines, protective circuits including non-linear elements, and a pixel portion in a display device. Over a substrate 10 having an insulating surface, scan lines 13 and signal lines 14 intersect with each other to form a pixel portion 17.

The pixel portion 17 includes a plurality of pixels 18 arranged in matrix. The pixel 18 includes a pixel transistor 19 connected to the scan line 13 and the signal line 14, a storage capacitor portion 20, and a pixel electrode 21.

In the pixel structure illustrated here, one electrode of the storage capacitor portion 20 is connected to the pixel transistor 19 and the other electrode is connected to a capacitor line 22. Moreover, the pixel electrode 21 forms one electrode which drives a display element (such as a liquid crystal element, a light-emitting element, or a contrast medium (electronic ink)). The other electrode of such a display element is connected to a common terminal 23.

A protective circuit is provided between the pixel portion 17, and a scan line input terminal 11 and a signal line input terminal 12. In Embodiment 1, a plurality of protective circuits is provided. Therefore, even though surge voltage due to static electricity and the like is applied to the scan line 13, the signal line 14, and a capacitor bus line 27, the pixel transistor 19 and the like are not broken. Accordingly, the protective circuit has a structure for releasing charge to a common wiring 29 or a common wiring 28 when surge voltage is applied to the protective circuit.

In Embodiment 1, a protective circuit 24 is provided on the scan line 13 side, a protective circuit 25 is provided on the signal line 14 side, and a protective circuit 26 is provided on the capacitor bus line 27 side. Needless to say, the structures of the protective circuits are not limited to those above.

FIG. 2 illustrates an example of the protective circuit. This protective circuit includes a non-linear element 30 and a non-linear element 31 which are arranged in parallel to each other with the scan line 13 interposed therebetween. Each of the non-linear element 30 and the non-linear element 31 includes a two-terminal element such as a diode or a three-terminal element such as a transistor. For example, the non-linear element can be formed through the same steps as the pixel transistor of the pixel portion. For example, characteristics similar to those of a diode can be achieved by connecting a gate terminal to a drain terminal of the non-linear element.

A first terminal (gate) and a third terminal (drain) of the non-linear element 30 are connected to the scan line 13, and a second terminal (source) thereof is connected to the common wiring 29. A first terminal (gate) and a third terminal (drain) of the non-linear element 31 are connected to the common wiring 29, and a second terminal (source) thereof is connected to the scan line 13. That is, the protective circuit illustrated in FIG. 2 includes two transistors whose rectifying directions are opposite to each other with respect to the scan line 13 and which connect the scan line 13 and the common wiring 29 to each other. In other words, between the scan line 13 and the common wiring 29, there are a transistor whose rectifying direction is from the scan line 13 to the common wiring 29 and a transistor whose rectifying direction is from the common wiring 29 to the scan line 13.

In the protective circuit illustrated in FIG. 2, in the case where the scan line is charged positively or negatively with respect to the common wiring 29 due to static electricity or the like, current flows in a direction that cancels the charge. For example, if the scan line 13 is positively charged, current flows in a direction in which the positive charge is released to the common wiring 29. Owing to this operation, the electrostatic breakdown or the shift in threshold voltage of the pixel transistor 19 connected to the charged scan line 13 can be prevented. Moreover, it is possible to prevent dielectric breakdown of the insulating film between the charged scan line 13 and another wiring that intersects with the charged scan line 13 with an insulating layer interposed therebetween.

Note that in FIG. 2, a pair of the non-linear element 30 whose first terminal (gate) is connected to the scan line 13 and the non-linear element 31 whose first terminal (gate) is connected to the common wiring 29 is used; that is, the rectifying directions of the non-linear element 30 and the non-linear element 31 are opposite to each other. The common wiring 29 and the scan line 13 are connected in parallel to each other via the second terminal (source) and the third terminal (drain) of each non-linear element. As another structure, a non-linear element may be further added in parallel connection, so that the operation stability of the protective circuit may be enhanced. For example, FIG. 3 illustrates a protective circuit including a non-linear element 30 a and a non-linear element 30 b, and a non-linear element 31 a and a non-linear element 31 b, which is provided between the scan line 13 and the common wiring 29. This protective circuit includes four non-linear elements in total: two non-linear elements (30 b and 31 b), a first terminal (gate) of each of which is connected to the common wiring 29 and two non-linear elements (30 a and 31 a), a first terminal (gate) of each of which is connected to the scan line 13. That is to say, two pairs of non-linear elements are connected between the common wiring 29 and the scan line 13, each pair including two non-linear elements provided so that their rectifying directions are opposite to each other. In other words, between the scan line 13 and the common wiring 29, there are two transistors the rectifying direction of each of which is from the scan line 13 to the common wiring 29 and two transistors the rectifying direction of each of which is from the common wiring 29 to the scan line 13. When the common wiring 29 and the scan line 13 are connected to each other with the four non-linear elements in this manner, it is possible to prevent, even if surge voltage is applied to the scan line 13 and moreover even if the common wiring 29 is charged by static electricity or the like, the charge from directly flowing through the scan line 13. Note that FIG. 28A illustrates an example in which four non-linear elements 740 a, 740 b, 740 c and 740 d are provided over a substrate and FIG. 28B is an equivalent circuit diagram thereof. Note that reference numerals 650 and 651 in FIGS. 28A and 29B denote a scan line and a common wiring, respectively.

FIG. 27A illustrates an example of providing a protective circuit which is formed using an odd number of non-linear elements over a substrate, and FIG. 27B is an equivalent circuit diagram thereof. In this circuit, a non-linear element 730 b and a non-linear element 730 a are connected to a non-linear element 730 c as switching elements. By the serial connection of the non-linear elements in this manner, instantaneous load applied to the non-linear elements of the protective circuit can be deconcentrated. Note that reference numerals 650 and 651 in FIGS. 27A and 27B denote a scan line and a common wiring, respectively.

FIG. 2 illustrates an example of the protective circuit which is provided on the scan line 13 side; however, a protective circuit with a similar structure can be provided on the signal line 14 side.

FIG. 4A is a plan view illustrating an example of a protective circuit and FIG. 4B is an equivalent circuit diagram thereof. FIG. 5 is a cross-sectional view taken along line Q1-Q2 of FIG. 4A. A structure example of the protective circuit is described below with reference to FIGS. 4A and 4B and FIG. 5.

The non-linear element 30 a and the non-linear element 30 b include a gate electrode 15 and a gate electrode 16, respectively, which are formed using the same layer as the scan line 13. A gate insulating layer 37 is formed over the gate electrode 15 and the gate electrode 16. A first wiring layer 38 and a second wiring layer 39 are provided over the gate insulating film 37 so as to face with each other over the gate electrode 15. Note that the non-linear element 30 a and the non-linear element 30 b have the same structure in the main portion.

A first oxide semiconductor layer 36 is provided so as to cover a region between the first wiring layer 38 and the second wiring layer 39 which face with each other. That is, the first oxide semiconductor layer 36 is provided so as to overlap with the gate electrode 15 and be in contact with the gate insulating layer 37, side face portions of conductive layers 41 in the first wiring layer 38 and the second wiring layer 39, and side face portions and part of top face portions of second oxide semiconductor layers 40 in the first wiring layer 38 and the second wiring layer 39. Here, the first wiring layer 38 and the second wiring layer 39 each have a structure in which the conductive layer 41 and the second oxide semiconductor layer 40 are stacked in that order from the gate insulating layer 37 side. The gate insulating layer 37 is formed from an oxide such as silicon oxide or aluminum oxide.

The first oxide semiconductor layer 36 has higher oxygen concentration than the second oxide semiconductor layer 40. In other words, the first oxide semiconductor layer 36 is oxygen-excess type, while the second oxide semiconductor layer 40 is oxygen-deficiency type. Since the donor-type defects can be reduced by increasing the oxygen concentration of the first oxide semiconductor layer 36, there are advantageous effects of longer carrier lifetime and higher mobility. On the other hand, when the oxygen concentration of the second oxide semiconductor layer 40 is made lower than that of the first oxide semiconductor layer 36, the carrier concentration can be increased and the second oxide semiconductor layer 40 can be utilized for forming a source region and a drain region.

As for the structure of the oxide semiconductor, the first oxide semiconductor layer 36 has an amorphous structure and the second oxide semiconductor layer 40 includes a nanocrystal in an amorphous structure in some cases. Then, the first oxide semiconductor layer 36 has a characteristic that the electrical conductivity thereof is lower than that of the second oxide semiconductor layer 40. Therefore, the second oxide semiconductor layers 40 used as the components of the first wiring layer 38 and the second wiring layer 39 in the non-linear element 30 a and the non-linear element 30 b of Embodiment 1 can have functions similar to those of a source region and a drain region of a transistor.

The first oxide semiconductor layer 36 and the second oxide semiconductor layer 40 are formed from zinc oxide (ZnO) typically, or an oxide semiconductor including In, Ga, and Zn.

The first oxide semiconductor layer 36 is provided in contact with the gate insulating layer 37. Moreover, the first oxide semiconductor layer 36 is provided in contact with the second oxide semiconductor layer 40 with higher electrical conductivity than the first oxide semiconductor layer 36. When the non-linear element 30 a and the non-linear element 30 b each have the structure as above where the oxide semiconductor layers having different physical properties as above are bonded to each other, stable operation becomes possible as compared with Schottky junction formed in the case where the first wiring layer 38 and the second wiring layer 39 are formed using only metal layers. That is, as compared with the case of using only metal wirings, the thermal stability is increased, so that the stable operation becomes possible. Accordingly, the function of the protective circuit is enhanced and stable operation can be achieved. Moreover, the amount of junction leakage can be reduced and the characteristics of the non-linear element 30 a and the non-linear element 30 b can be improved.

When first wiring layer 38 and the second wiring layer 39 each have a structure in which the second oxide semiconductor layer 40 is provided over the conductive layer 41 formed from a metal material and the first oxide semiconductor layer 36 is in contact with the top surface of the second oxide semiconductor layer 40, the area at the junction portion is increased, so that current easily flows through the non-linear element 30 a. Therefore, in the case where the non-linear element 30 a is used for a protective circuit, even though surge voltage is applied to a signal line and the like, charge can be discharged to a common wiring rapidly.

An interlayer insulating layer 42 is provided over the first oxide semiconductor layer 36. The interlayer insulating layer 42 is formed from an oxide such as silicon oxide or aluminum oxide. When silicon nitride, aluminum nitride, silicon oxynitride, or aluminum oxynitride is stacked over silicon oxide or aluminum oxide, the function as the protective film can be enhanced.

In any case, when the interlayer insulating layer 42 being in contact with the first oxide semiconductor layer 36 is an oxide, it is possible to prevent oxygen from being extracted from the first oxide semiconductor layer 36 and prevent the first oxide semiconductor layer 36 from changing into an oxygen-deficiency type. Moreover, by the structure where the first oxide semiconductor layer 36 is not in direct contact with an insulating layer including nitride, it is possible to prevent hydrogen in the nitride from diffusing and causing defects in the first oxide semiconductor layer 36 due to a hydroxyl group or the like.

The interlayer insulating layer 42 is provided with a contact hole 43 where the scan line 13 formed using the same layer as the gate electrode 15 is connected to a third terminal (drain) of the non-linear element 30 a. This connection is made by a third wiring layer 44 formed from the same material as the pixel electrode of the pixel portion. The third wiring layer 44 is formed from a material which is used for forming a transparent electrode, for example, from indium tin oxide (ITO), zinc oxide (ZnO), tin oxide (SnO₂), or the like. Thus, the third wiring layer 44 has higher resistance than a wiring formed from a metal material. When the protective circuit includes the wirings including such a resistance component, it is possible to prevent an excessive amount of current from flowing through the non-linear element 30 a and the non-linear element 30 a from being destroyed.

Although FIGS. 4A and 4B and FIG. 5 illustrate the example of the protective circuit provided at the scan line 13, a similar protective circuit can be applied to a signal line, a capacitor bus line, or the like.

According to Embodiment 1, by the provision of the protective circuit including the non-linear element including the oxide semiconductor in this manner, a display device having a structure which is suitable as a protective circuit can be provided. Then, the function of the protective circuit can be enhanced and the operation can be stabilized by the use of the non-linear element including an oxide semiconductor.

Embodiment 2

In Embodiment 2, an embodiment of a process for manufacturing the protective circuit illustrated in FIG. 4A in Embodiment 1 is described with reference to FIGS. 6A to 6C and FIGS. 7A to 7C. FIGS. 6A to 6C and FIGS. 7A to 7C are cross-sectional views taken along line Q1-Q2 of FIG. 4A.

In FIG. 6A, a glass substrate of barium borosilicate glass, aluminoborosilicate glass, aluminosilicate glass, or the like available in the market can be used as the substrate 100 having a light-transmitting property. For example, a glass substrate which includes more barium oxide (BaO) than boric acid (B₂O₃) in composition ratio and whose strain point is 730° C. or higher is preferable. This is because the glass substrate is not strained even in the case where the oxide semiconductor layer is thermally processed at high temperatures of about 700° C.

Next, a conductive layer is formed entirely over the substrate 100. After that, a resist mask is formed by a first photolithography process, and an unnecessary portion is removed by etching to form wirings and an electrode (such as a gate wiring including a gate electrode 101, a capacitor wiring, and a terminal). At this time, the etching is performed so that at least an end portion of the gate electrode 101 is tapered.

The gate wiring including the gate electrode 101, the capacitor wiring, and the terminal of a terminal portion are desirably formed from a low-resistance conductive material such as aluminum (Al) or copper (Cu); however, since aluminum alone has disadvantages such as low heat resistance and a tendency to be corroded, it is used in combination with a conductive material having heat resistance. As the conductive material having heat resistance, an element selected from titanium (Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr), and neodymium (Nd), an alloy containing the above element as its component, an alloy film in which some of the above elements are combined, or a nitride containing the above element as its component may be used. FIG. 6A is a cross-sectional view at this stage.

Subsequently, a gate insulating layer 102 is formed entirely over the gate electrode 101. The gate insulating layer 102 is formed by a sputtering method or the like to a thickness of 50 to 250 nm.

For example, a silicon oxide film is formed by a sputtering method to a thickness of 100 nm as the gate insulating layer 102. Needless to say, the gate insulating layer 102 is not limited to such a silicon oxide film and may be a single layer or a stack of layers including another insulating film, such as a silicon oxynitride film, a silicon nitride film, an aluminum oxide film, or a tantalum oxide film.

Next, a conductive film is formed from a metal material over the gate insulating layer 102 by a sputtering method or a vacuum evaporation method. As the material of the conductive layer, there are an element selected from Al, Cr, Ta, Ti, Mo, and W, an alloy including the above element, an alloy film in which some of the above elements are combined, and the like. Here, the conductive film has a three-layer structure in which a Ti film is formed, an aluminum (Al) film is stacked over the Ti film, and another Ti film is stacked over the Al film. Alternatively, the conductive film may have a two-layer structure in which a Ti film is stacked over an Al film. Further alternatively, the conductive film may have a single-layer structure of an aluminum film including silicon or a titanium film.

Next, a second oxide semiconductor film is formed over the gate insulating layer 102 by a sputtering method. Here, sputtering deposition is performed under the condition where a target includes indium oxide (In₂O₃), gallium oxide (Ga₂O₃), and zinc oxide (ZnO) at a composition ratio of 1:1:1 (=In₂O₃:Ga₂O₃:ZnO), the pressure in a deposition chamber is set at 0.4 Pa, the electric power is set at 500 W, the deposition temperature is set to room temperature, and the argon gas flow rate is set at 40 sccm. Thus, a semiconductor film including In, Ga, Zn, and oxygen is formed as the second oxide semiconductor film. Although the target where the composition ratio is In₂O₃:Ga₂O₃:ZnO=1:1:1 is used intentionally, an oxide semiconductor film including a crystal grain which has a size of 1 nm to 10 nm just after the deposition is often obtained. It can be said that the presence or absence of crystal grains and the density of crystal grains can be controlled and the diameter of the crystal grain can be adjusted within 1 nm to 10 nm, all by adjusting as appropriate, the deposition condition of reactive sputtering, such as the target composition ratio, the deposition pressure (0.1 Pa to 2.0 Pa), the electric power (250 W to 3000 W: 8 inchesφ), the temperature (room temperature to 100° C.), and the like. The thickness of the second oxide semiconductor film is set to 5 nm to 20 nm. Needless to say, in the case where the film includes crystal grains, the size of the crystal grain does not exceed the film thickness. In Embodiment 2, the second oxide semiconductor film has a thickness of 5 nm.

The gate insulating layer, the conductive film, and the second oxide semiconductor film can be formed by a sputtering method successively without exposure to the air by changing the gas introduced to the chamber and the target set in the chamber as appropriate. The successive deposition without exposure to the air can prevent impurity mixture. In the case of successive deposition without exposure to the air, a manufacturing apparatus of multichamber type is preferable.

Next, a second photolithography process is performed to form a resist mask, and the second oxide semiconductor film is etched. Here, wet etching is performed using ITO07N (product of Kanto Chemical Co., Inc.) to remove an unnecessary portion; thus, second oxide semiconductor layers 111 a and 111 b are formed. Note that the etching here is not limited to wet etching and may be dry etching.

Next, the resist mask used in the step of etching the second oxide semiconductor film is used to remove an unnecessary portion of the conductive film over the gate insulating layer by etching, whereby a source electrode layer 105 a and a drain electrode layer 105 b are formed. The etching may be wet etching or dry etching. Here, dry etching is employed using a mixed gas of SiCl₄, Cl₂, and BCl₃ to etch the conductive film in which the Ti film, the Al film, and the Ti film are stacked. Thus, the source electrode layer 105 a and the drain electrode layer 105 b are formed. Note that a cross-sectional view after the resist mask is removed is shown in FIG. 6B.

Next, plasma treatment is performed. Here, reverse sputtering where plasma is generated after introduction of an oxygen gas and an argon gas into a deposition chamber is performed, so that the exposed gate insulating layer is irradiated with oxygen radicals or oxygen. Thus, dust adhering to the surface is removed and moreover the surface of the gate insulating layer is modified into an oxygen-excess region. It is effective to perform the oxygen radical treatment on the surface of the gate insulating layer so that the surface is made into an oxygen-excess region, because an oxygen supply source for modifying the interface of the first oxide semiconductor layer is made in thermal treatment (200° C. to 600° C.) for increasing the reliability in a later step. A cross-sectional view when this step is completed is shown in FIG. 6C.

Note that an oxide film (not shown) is formed at exposed side surfaces of the source electrode layer 105 a and the drain electrode layer 105 b depending on the condition of the plasma treatment; however, this does not lead to a problem because the source electrode layer 105 a and the drain electrode layer 105 b are in direct contact with a channel formation region in this structure of Embodiment 2. Rather, by the formation of this oxide film, the source electrode layer 105 a and the drain electrode layer 105 b are electrically connected to the channel formation region with a source region and a drain region, each of which is formed using the second oxide semiconductor layer, interposed therebetween. Further, since the plasma treatment is performed after the source region and the drain region, each of which includes the second oxide semiconductor layer, are formed over the source electrode layer and the drain electrode layer, only the exposed end portions of the source electrode layer and the drain electrode layer are oxidized. Since the other regions are not oxidized, the source electrode layer and the drain electrode layer can be kept low-resistant. Moreover, since the area where the first semiconductor layer is in contact with the source region and the drain region, each of which includes the second oxide semiconductor layer, is large, the source region or the drain region can be electrically connected to the semiconductor layer favorably.

Next, the first oxide semiconductor film is formed in such a manner that the substrate on which the plasma treatment has been performed is not exposed to the air. The first oxide semiconductor film formed in such a manner that the substrate on which the plasma treatment has been performed is not exposed to the air can avoid the trouble that dust or moisture adheres to the interface between the gate insulating layer and the semiconductor film. Here, the first oxide semiconductor film is formed in an oxygen atmosphere under the condition where the target is an oxide semiconductor target including In, Ga, and Zn (composition ratio is In₂O₃:Ga₂O₃:ZnO=1:1:1) with a diameter of 8 inches, the distance between the substrate and the target is set at 170 mm, the pressure is set at 0.4 Pa, and the direct current (DC) power supply is set at 0.5 kW. Note that a pulse direct current (DC) power supply is preferable because dust can be reduced and the film thickness can be uniform. The thickness of the first oxide semiconductor layer is set to 5 nm to 200 nm. The thickness of the first oxide semiconductor film in Embodiment 2 is 100 nm.

When the first oxide semiconductor film is formed under the different condition from the second oxide semiconductor film, the first oxide semiconductor film has different composition from the second oxide semiconductor film; for example, the first oxide semiconductor film includes more oxygen than the second oxide semiconductor film. In this case, for example, the ratio of the oxygen gas flow rate to the argon gas flow rate in the deposition condition of the first oxide semiconductor film is set higher than that of the second oxide semiconductor film. Specifically, the second oxide semiconductor film is formed in a rare gas (such as argon or helium) atmosphere (or a gas including oxygen at 10% or less and argon at 90% or more), while the first oxide semiconductor film is formed in an oxygen atmosphere (or a mixed gas of oxygen and argon with the flow rate of oxygen being more than that of argon). When the first oxide semiconductor film includes more oxygen than the second oxide semiconductor film, the first oxide semiconductor film can have lower electrical conductivity than the second oxide semiconductor film. Moreover, when the first oxide semiconductor film includes a large amount of oxygen, the amount of off current can be reduced; therefore, a thin film transistor with a high on/off ratio can be provided.

The first oxide semiconductor film may be formed in the same chamber as the chamber where the reverse sputtering is performed previously, or may be formed in a different chamber from the chamber where the reverse sputtering is performed previously as long as the deposition can be performed without exposure to the air.

Next, thermal treatment at 200° C. to 600° C., typically 300° C. to 500° C., is preferably performed. Here, thermal treatment is performed in a furnace at 350° C. for an hour in a nitrogen atmosphere. This thermal treatment allows atoms of the IGZO semiconductor films to be rearranged. Since the distortion that interrupts carrier movement is released by this thermal treatment, the thermal treatment here (including photo-annealing) is important. There is no particular limitation on when to perform the thermal treatment as long as it is performed after the formation of the first oxide semiconductor film; for example, it is performed after the formation of the pixel electrode.

Next, a third photolithography process is performed to form a resist mask, and an unnecessary part is removed by etching. Thus, a first oxide semiconductor layer 103 is formed. Here, wet etching is performed using ITO07N (product of Kanto Chemical Co., Inc.); thus, the first oxide semiconductor layer 103 is formed. Note that since the first oxide semiconductor film and the second oxide semiconductor film are dissolved in the same etchant, the etching performed here remove part of the second oxide semiconductor film. That is to say, part of the second oxide semiconductor film (IGZO semiconductor film) which is covered with the resist mask and the first oxide semiconductor film is protected; however, the exposed part of the second oxide semiconductor film is etched, thereby forming a source region 104 a and a drain region 104 b. Note that the etching of the first oxide semiconductor layer 103 is not limited to wet etching and may be dry etching. Then, the resist mask is removed. Through these steps, the non-linear element 30 a in which the first oxide semiconductor layer 103 is a channel formation region is completed. A cross-sectional view at this point is shown in FIG. 7A.

Next, a protective insulating film 107 covering the non-linear element 30 a is formed. The protective insulating film 107 can be formed using a silicon nitride film, a silicon oxide film, a silicon oxynitride film, an aluminum oxide film, a tantalum oxide film, or the like by a sputtering method or the like.

Next, a fourth photolithography process is performed to form a resist mask, and the protective insulating film 107 is etched. Thus, a contact hole 125 that reaches the drain electrode layer 105 b is formed. Moreover, it is preferable to etch the gate insulating layer 102 by using the same resist mask so as to form a contact hole 126 that reaches the gate electrode because the number of photomasks can be reduced. The resist mask is removed, and a cross-sectional view at this point is shown in FIG. 7B.

Then, a third wiring layer 128 is formed. When a transparent conductive film is used, the pixel electrode can be formed together with the third wiring layer 128. As a material for the transparent conductive film, indium oxide (In₂O₃), indium oxide-tin oxide alloy (In₂O₃—SnO₂, abbreviated to ITO), or the like can be given, and it can be formed by a sputtering method, a vacuum evaporation method, or the like. Etching treatment of such materials is performed using a chlorinated acid based solution. However, since etching of ITO particularly tends to leave residue, an alloy of indium oxide and zinc oxide (In₂O₃—ZnO) may be used in order to improve etching processability. Thus, the transparent conductive film is etched in this manner to form the third wiring layer 128.

Next, a fifth photolithography process is performed to form a resist mask, and an unnecessary portion of the transparent conductive film is removed. Thus, a pixel electrode is formed in a pixel portion which is not illustrated.

Moreover, in this fifth photolithography process, a capacitor wiring and the pixel electrode together form a storage capacitor in a capacitor portion, which is not illustrated, by using the gate insulating layer 102 and the protective insulating film 107 as dielectrics.

Moreover, in this fifth photolithography process, the resist mask covers a terminal portion, so that the transparent conductive film formed in the terminal portion is left. The transparent conductive film serves as an electrode or a wiring used for connection with an FPC, a terminal electrode for connection which functions as an input terminal of a source wiring, or the like.

Moreover, in Embodiment 2, the drain electrode layer 105 b of the non-linear element 30 a is connected to the scan line 108 in the contact holes 125 and 126 via the third wiring layer 128 formed using the transparent conductive film.

Then, the resist mask is removed. A cross-sectional view at this point is shown in FIG. 7C.

Through the five photolithography processes performed in the above manner, the protective circuit having the plurality of non-linear elements (in Embodiment 2, the two non-linear elements 30 a and 30 b) can be completed by using the five photomasks. According to Embodiment 2, a plurality of TFTs can be completed by a similar method together with the non-linear elements. Therefore, a pixel portion including bottom-gate n-channel TFTs and a protective circuit can be manufactured at the same time. In other words, a board for an active matrix display device, on which a protective diode is mounted, can be manufactured in accordance with the steps described in Embodiment 2.

Embodiment 3

In Embodiment 3, the protective circuit illustrated in FIG. 4A in Embodiment 1 is formed using a non-linear element with a different structure from that described in Embodiment 2. That is, in a non-linear element of this example, source regions and drain regions are provided above and below a source electrode layer and a drain electrode layer. A thin film transistor having a different structure from that of Embodiment 2 and its manufacturing method are described with reference to FIGS. 8A to 8C and FIGS. 9A to 9C.

In Embodiment 3, the same portions as those of FIGS. 6A to 6C and FIGS. 7A to 7C are denoted with the same reference numerals and the description of the same steps is not made because Embodiment 3 is only partly different from Embodiment 1.

First, in a manner similar to Embodiment 2, a conductive layer is formed over the substrate 100 and then a first photolithography process is performed to form a resist mask, and an unnecessary portion is removed by etching. Thus, wirings and an electrode (a gate wiring including the gate electrode 101, a capacitor wiring, and a first terminal) are formed. A cross-sectional view at this point is shown in FIG. 8A.

Next, in a manner similar to Embodiment 2, the gate insulating layer 102 is formed entirely over the gate electrode 101. The gate insulating layer 102 is formed by a sputtering method to a thickness of 50 nm to 250 nm. For example, a silicon oxide film is formed as the gate insulating layer 102 by a sputtering method to a thickness of 110 nm.

Next, a third oxide semiconductor film is formed over the gate insulating layer 102 by a sputtering method. Here, sputtering is performed under the condition where a target includes indium oxide (In₂O₃), gallium oxide (Ga₂O₃), and zinc oxide (ZnO) at a composition ratio of 1:1:1 (=In₂O₃:Ga₂O₃:ZnO), the pressure is set at 0.4 Pa, the electric power is set at 500 W, the deposition temperature is set to room temperature, and the argon gas flow rate is set at 40 sccm. Although the target where the composition ratio is In₂O₃:Ga₂O₃:ZnO=1:1:1 is used intentionally, an IGZO semiconductor film including a crystal grain which has a size of 1 nm to 10 nm just after the deposition is often obtained. It can be said that the presence or absence of crystal grains and the density of crystal grains can be controlled and the diameter of the crystal grain can be adjusted within 1 nm to 10 nm, all by adjusting as appropriate, the deposition condition of reactive sputtering, such as target composition ratio, the deposition pressure (0.1 Pa to 2.0 Pa), the electric power (250 W to 3000 W: 8 inchesφ), the temperature (room temperature to 100° C.), and the like. The thickness of the third oxide semiconductor film is set to 5 nm to 20 nm. Needless to say, in the case where the film includes crystal grains, the size of the crystal grain does not exceed the film thickness. In Embodiment 3, the third oxide semiconductor film has a thickness of 5 nm.

Next, a conductive film is formed from a metal material over the third oxide semiconductor film by a sputtering method or a vacuum evaporation method. As the material of the conductive film, there are an element selected from Al, Cr, Ta, Ti, Mo, and W, an alloy including the above element, an alloy film in which some of the above elements are combined, and the like. Here, the conductive film has a single-layer structure of an aluminum film including silicon. Alternatively, the conductive film may have a stacked structure in which a titanium film is stacked over an aluminum film. Further alternatively, the conductive film may have a three-layer structure in which a Ti film is formed, an aluminum (Al) film is stacked over the Ti film, and another Ti film is stacked over the Al film.

Next, a second oxide semiconductor film is formed over the conductive film by a sputtering method. This second oxide semiconductor film can be formed under the same deposition condition as that of the third oxide semiconductor film. In the second oxide semiconductor film formed under a condition similar to that of the third oxide semiconductor film, an IGZO semiconductor film including a crystal grain with a size of 1 nm to 10 nm is formed in some cases just after the deposition. The thickness of the second oxide semiconductor film is set to 5 nm to 20 nm. In Embodiment 3, the thickness of the second oxide semiconductor film is 5 nm.

The gate insulating layer, the third oxide semiconductor film, the conductive film, and the second oxide semiconductor film can be formed by a sputtering method successively without exposure to the air by changing the gas introduced to the chamber and the target set in the chamber as appropriate. The successive deposition without exposure to the air can prevent impurity mixture. In the case of successive deposition without exposure to the air, a manufacturing apparatus of multichamber type is preferable.

Next, a second photolithography process is performed to form a resist mask over the second oxide semiconductor film, and unnecessary portions of the third oxide semiconductor layer, the conductive film, and the second oxide semiconductor film are removed by etching. Thus, the first source region 106 a and the first drain region 106 b, the source electrode layer 105 a and the drain electrode layer 105 b, and the second oxide semiconductor layers 111 a and 111 b are formed. The etching may be wet etching or dry etching. Here, wet etching is performed using ITO07N (product of Kanto Chemical Co., Inc.) to form the second oxide semiconductor layers 111 a and 111 b, and then dry etching is performed using as a reactive gas, a mixed gas of SiCl₄, Cl₂, and BCl₃ to etch the conductive film including the aluminum film including silicon. Thus, the source electrode layer 105 a and the drain electrode layer 105 b are formed. After that, the same resist mask is used to perform wet etching using ITO07N (product of Kanto Chemical Co., Inc.), whereby the first source region 106 a and the first drain region 106 b are formed. A cross-sectional view after the resist mask is removed is shown in FIG. 8B.

Moreover, although not shown, part of the IGZO semiconductor film which overlaps with the capacitor wiring in the capacitor portion is removed. In the terminal portion, the third oxide semiconductor layer remains.

Next, plasma treatment is performed. Here, reverse sputtering where plasma is generated by introduction of an oxygen gas and an argon gas into a deposition chamber is performed, so that the exposed gate insulating layer is irradiated with oxygen radicals or oxygen. Thus, dust adhering to the surface is removed and moreover the surface of the gate insulating layer is modified into an oxygen-excess region. It is effective to perform the plasma treatment on the surface of the gate insulating layer so that the surface is made into an oxygen-excess region, in point of that an oxygen source for modifying the interface of the first oxide semiconductor layer is made in thermal treatment (200° C. to 600° C.) for increasing the reliability in a later step. A cross-sectional view when this step is completed is shown in FIG. 8C.

Since the first source region 106 a and the first drain region 106 b are provided below the source electrode layer 105 a and the drain electrode layer 105 b, the plasma damage on the first source region 106 a and the first drain region 106 b can be reduced. Moreover, the second oxide semiconductor layers 111 a and 111 b are provided over the source electrode layer 105 a and the drain electrode layer 105 b. Therefore, the increase in wiring resistance due to the oxidization of the source electrode layer 105 a and the drain electrode layer 105 b can be suppressed.

Note that an oxide film (not shown) is formed at exposed side surfaces of the source electrode layer 105 a and the drain electrode layer 105 b depending on the condition of the plasma treatment; however, this does not lead to a problem because the source electrode layer 105 a and the drain electrode layer 105 b are not in direct contact with a channel formation region in this structure of Embodiment 3. Rather, by the formation of this oxide film, the source electrode layer 105 a and the drain electrode layer 105 b are electrically connected to the channel formation region with the source region and the drain region interposed therebetween.

Next, the first oxide semiconductor film is formed in such a manner that the substrate on which the plasma treatment has been performed is not exposed to the air. The first oxide semiconductor film formed in such a manner that the substrate on which the plasma treatment has been performed is not exposed to the air can avoid the trouble that dust or moisture adheres to the interface between the gate insulating layer and the semiconductor film. Here, the first oxide semiconductor film is formed in an oxygen atmosphere under the condition where the target is an oxide semiconductor target including In, Ga, and Zn (composition ratio is In₂O₃:Ga₂O₃:ZnO=1:1:1) with a diameter of 8 inches, the distance between the substrate and the target is set at 170 mm, the pressure is set at 0.4 Pa, and the direct current (DC) power supply is set at 0.5 kW. Note that a pulse direct current (DC) power supply is preferable because dust can be reduced and the film thickness can be uniform. The thickness of the first oxide semiconductor film is set to 5 nm to 200 nm. The thickness of the first oxide semiconductor film in Embodiment 3 is 100 nm.

When the first oxide semiconductor film is formed under a condition different from those of the second and third oxide semiconductor films, the first oxide semiconductor film has a different composition from those of the second and third oxide semiconductor films; for example, the first oxide semiconductor film can have higher oxygen concentration than the second and third oxide semiconductor films. In this case, for example, the first oxide semiconductor film is formed under the condition where the proportion of oxygen gas in the atmosphere is higher than that of the second and third oxide semiconductor films.

Specifically, the second and third oxide semiconductor films are formed in a rare gas (such as argon or helium) atmosphere (or a gas including oxygen at 10% or less and argon at 90% or more), while the first oxide semiconductor film is formed in an oxygen atmosphere (or a mixed gas of oxygen and argon with the flow rate of oxygen being more than that of argon and the ratio therebetween being 1:1 or more).

When the IGZO semiconductor film serving as the first oxide semiconductor film includes more oxygen than the IGZO semiconductor films serving as the second and third oxide semiconductor films, the IGZO semiconductor film serving as the first oxide semiconductor film can have lower electrical conductivity than the IGZO semiconductor films serving as the second and third oxide semiconductor films. Moreover, when the first oxide semiconductor film includes a large amount of oxygen, the amount of off current can be reduced; therefore, a thin film transistor with a high on/off ratio can be provided.

The first oxide semiconductor film may be formed in the same chamber as the chamber where the reverse sputtering is performed previously, or may be formed in a different chamber from the chamber where the reverse sputtering is performed previously as long as the deposition can be performed without exposure to the air.

Next, thermal treatment at 200° C. to 600° C., typically 300° C. to 500° C., is preferably performed. Here, thermal treatment is performed in a furnace at 350° C. for an hour in a nitrogen atmosphere. This thermal treatment allows atoms of the IGZO semiconductor films to be rearranged. Since the distortion that interrupts carrier movement is released by this thermal treatment, the thermal treatment here (including photo-annealing) is important. There is no particular limitation on when to perform the thermal treatment as long as it is performed after the formation of the first oxide semiconductor film; for example, it is performed after the formation of the pixel electrode.

Next, a third photolithography process is performed to form a resist mask, and an unnecessary portion is removed by etching. Thus, the first oxide semiconductor layer 103 is formed. Here, wet etching is performed using ITO07N (product of Kanto Chemical Co., Inc.) to remove an unnecessary portion of the first oxide semiconductor film; thus, the first oxide semiconductor layer 103 is formed. Note that since the first oxide semiconductor film, the second oxide semiconductor film, and the third oxide semiconductor film are dissolved in the same etchant, the etching performed here remove part of the second oxide semiconductor film and part of the third oxide semiconductor film. Parts of the second oxide semiconductor film, which are covered with the resist mask and the first oxide semiconductor film and are therefore left, serves as a second source region 104 a and a second drain region 104 b. One side surface of the third oxide semiconductor film, which is covered with the first oxide semiconductor film, is protected; however, as illustrated in FIG. 9A, the other side surface of the third oxide semiconductor film is exposed. Therefore, the other side surface is slightly etched, so that the shape of the end face changes. Note that the etching of the first oxide semiconductor layer 103 is not limited to wet etching and may be dry etching. Through these steps, the non-linear element 30 a in which the first oxide semiconductor layer 103 is a channel formation region is completed. A cross-sectional view at this point is shown in FIG. 9A.

Next, the protective insulating film 107 covering the non-linear element 30 a is formed. Since the steps after this are the same as those of Embodiment 2, the description is made simply.

After the protective insulating film 107 is formed, a fourth photolithography process is performed to form a resist mask, and the protective insulating film 107 is etched. Thus, the contact holes 125 and 126 are formed. A cross-sectional view after the resist mask is removed is shown in FIG. 9B.

Next, after a transparent conductive film is formed, a fifth photolithography process is performed to form a resist mask. An unnecessary portion of the transparent conductive film is removed by etching, so that a pixel electrode which is not shown is formed.

Moreover, in Embodiment 3, the drain electrode layer 105 b of the non-linear element 30 a is connected to the scan line 108 in the contact holes 125 and 126 by the third wiring layer 128 formed using the transparent conductive film. A cross-sectional view at this point is shown in FIG. 9C.

Through the five photolithography processes performed in the above manner, the protective circuit having the plurality of non-linear elements (in Embodiment 3, the two non-linear elements 30 a and 30 b) can be completed by using the five photomasks. According to Embodiment 3, a plurality of TFTs can be completed by a similar method together with the non-linear elements. Therefore, a pixel portion including bottom-gate n-channel TFTs and a protective circuit can be manufactured at the same time. In other words, a board for an active matrix display device, on which a protective diode is mounted, can be manufactured in accordance with the steps described in Embodiment 3.

The adhesion between the gate insulating layer 102, and the source region 106 a and the drain region 106 b which are formed using the third oxide semiconductor layer is favorable and the thin films do not easily peel off. In other words, it is possible to prevent defects of the protective circuit due to the peeling of the thin films because the adhesion with the source electrode layer 105 a and the drain electrode layer 105 b is increased as compared with the case where a metal wiring of aluminum or the like is formed in direct contact with the gate insulating layer 102.

Embodiment 4

Embodiment 4 illustrates an example of electronic paper in which a protective circuit and a TFT in a pixel portion are provided over one substrate, as a display device to which an embodiment of the present invention is applied.

FIG. 10 illustrates active matrix type electronic paper as an example of a display device to which an embodiment of the present invention is applied. A thin film transistor 581 used for a display device can be manufactured in a manner similar to the non-linear element described in Embodiment 2. The thin film transistor 581 has high electrical characteristics and includes a gate insulating layer on which plasma treatment has been performed, a source region and a drain region which are formed using an IGZO semiconductor film of oxygen-deficiency type, a source electrode layer and a drain electrode layer which are in contact with the source region and the drain region, respectively, and an IGZO semiconductor layer of oxygen-excess type which is in contact with the source region and the drain region.

The electronic paper in FIG. 10 is an example of a display device in which a twisting ball display system is employed. The twisting ball display system refers to a method in which spherical particles each colored in black and white are arranged between a first electrode layer and a second electrode layer which are electrode layers used for a display element, and a potential difference is generated between the first electrode layer and the second electrode layer to control orientation of the spherical particles, so that display is performed.

The thin film transistor 581 has a bottom-gate structure in which the source electrode layer or the drain electrode layer is electrically connected to a first electrode layer 587 in an opening formed in an insulating layer 585. Between the first electrode layer 587 and a second electrode layer 588, spherical particles 589 are provided. Each spherical particle 589 includes a black region 590 a, a white region 590 b, and a cavity 594 filled with liquid around the black region 590 a and the white region 590 b. The circumference of the spherical particle 589 is filled with filler 595 such as a resin or the like (see FIG. 10). Note that reference numerals 580, 583, 584 and 596 in FIG. 10 denote a substrate, interlayer insulating layer, a protective film, and a substrate, respectively.

Further, instead of the twisting ball, an electrophoretic element can be used. A microcapsule having a diameter of about 10 μm to 200 μm, which is filled with transparent liquid, positively-charged white microparticles and negatively-charged black microparticles, is used. In the microcapsule which is provided between the first electrode layer and the second electrode layer, when an electric field is applied by the first electrode layer and the second electrode layer, the white microparticles and the black microparticles move to opposite sides to each other, so that white or black can be displayed. A display element using this principle is an electrophoretic display element, and is called electronic paper in general. The electrophoretic display element has higher reflectance than a liquid crystal display element, and thus, an assistant light is unnecessary. Moreover, power consumption is low and a display portion can be recognized in a dusky place. Furthermore, an image which is displayed once can be retained even when power is not supplied to the display portion. Accordingly, a displayed image can be stored even though a semiconductor device having a display function (which is also referred to simply as a display device or a semiconductor device provided with a display device) is distanced from an electric wave source which serves as a power supply.

According to Embodiment 4, a display device having a structure suitable as a protective circuit can be provided by forming the protective circuit with use of the non-linear element including the oxide semiconductor. In the connection structure between the first oxide semiconductor layer of the non-linear element and the wiring layers, the provision of the region which is bonded with the second oxide semiconductor layer, which has higher electrical conductivity than the first oxide semiconductor layer, allows stable operation as compared with the case of using only metal wirings. Accordingly, the function of the protective circuit is enhanced and the operation can be made stable. In this manner, according to Embodiment 4, electronic paper with high reliability as a display device can be completed. Moreover, by the structure similar to that of Embodiment 3, it is possible to manufacture a highly-reliable display device incorporating a protective circuit including the non-linear elements in which defects due to the peeling of the thin films are not easily caused.

Embodiment 4 can be implemented in combination with the structure described in Embodiment 1 as appropriate.

Embodiment 5

Embodiment 5 describes an example of manufacturing at least a protective circuit, part of a driver circuit, and a thin film transistor of a pixel portion over one substrate in a display device which is an example of a semiconductor device according to an embodiment of the present invention.

The thin film transistor in the pixel portion is formed in a manner similar to the non-linear element described in Embodiment 2 or 3. The thin film transistor is formed to be an n-channel TFT; therefore, part of a driver circuit which can be formed using an n-channel TFT is formed over the same substrate as the thin film transistor in the pixel portion.

FIG. 11A illustrates an example of a block diagram of an active matrix liquid crystal display device which is an example of a semiconductor device according to an embodiment of the present invention. The display device illustrated in FIG. 11A includes over a substrate 5300, a pixel portion 5301 including a plurality of pixels each provided with a display element; a scan line driver circuit 5302 that selects each pixel; and a signal line driver circuit 5303 that controls a video signal input to a selected pixel.

The pixel portion 5301 is connected to the signal line driver circuit 5303 with a plurality of signal lines S1 to Sm (not shown) extending in a column direction from the signal line driver circuit 5303 and connected to the scan line driver circuit 5302 with a plurality of scan lines G1 to Gn (not shown) extending in a row direction from the scan line driver circuit 5302. The pixel portion 5301 includes a plurality of pixels (not shown) arranged in matrix corresponding to the signal lines S1 to Sm and the scan lines G1 to Gn. In addition, each of the pixels is connected to a signal line Sj (any one of the signal lines S1 to Sm) and a scan line Gi (any one of the scan lines G1 to Gn).

The thin film transistor can be formed as an n-channel TFT by a method similar to that of the non-linear element described in Embodiment 2 or 3, and a signal line driver circuit including an n-channel TFT is described with reference to FIG. 12.

The signal line driver circuit in FIG. 12 includes a driver IC 5601, switch groups 5602_1 to 5602_M, a first wiring 5611, a second wiring 5612, a third wiring 5613, and wirings 5621_1 to 5621_M. Each of the switch groups 5602_1 to 5602_M includes a first thin film transistor 5603 a, a second thin film transistor 5603 b, and a third thin film transistor 5603 c.

The driver IC 5601 is connected to the first wiring 5611, the second wiring 5612, the third wiring 5613, and the wirings 5621_1 to 5621_M. Each of the switch groups 5602_1 to 5602_M is connected to the first wiring 5611, the second wiring 5612, the third wiring 5613, and one of the wirings 5621_1 to 5621_M corresponding to the switch groups 5602_1 to 5602_M, respectively. Each of the wirings 5621_1 to 5621_M is connected to three signal lines through the first thin film transistor 5603 a, the second thin film transistor 5603 b, and the third thin film transistor 5603 c. For example, the wiring 5621_J of the J-th column (one of the wirings 5621_1 to 5621_M) is connected to a signal line Sj−1, a signal line Sj, and a signal line Sj+1 through the first thin film transistor 5603 a, the second thin film transistor 5603 b, and the third thin film transistor 5603 c of the switch group 5602_J.

Note that a signal is input to each of the first wiring 5611, the second wiring 5612, and the third wiring 5613.

Note that the driver IC 5601 is preferably formed on a single-crystal substrate. The switch groups 5602_1 to 5602_M are preferably formed over the same substrate as the pixel portion. Therefore, the driver IC 5601 is preferably connected to the switch groups 5602_1 to 5602_M through an FPC or the like.

Next, operation of the signal line driver circuit in FIG. 12 is described with reference to a timing chart of FIG. 13. FIG. 13 illustrates the timing chart where a scan line Gi in the i-th row is selected. A selection period of the scan line Gi in the i-th row is divided into a first sub-selection period T1, a second sub-selection period T2, and a third sub-selection period T3. In addition, the signal line driver circuit in FIG. 12 operates similarly to FIG. 13 even when a scan line of another row is selected.

Note that the timing chart in FIG. 13 shows the case where the wiring 5621_J in the J-th column is connected to the signal line Sj−1, the signal line Sj, and the signal line Sj+1 through the first thin film transistor 5603 a, the second thin film transistor 5603 b, and the third thin film transistor 5603 c.

The timing chart of FIG. 13 shows timing when the scan line Gi in the i-th row is selected, timing 5703 a when the first thin film transistor 5603 a is turned on/off, timing 5703 b when the second thin film transistor 5603 b is turned on/off, timing 5703 c when the third thin film transistor 5603 c is turned on/off, and a signal 5721_J input to the wiring 5621_J in the J-th column.

In the first sub-selection period T1, the second sub-selection period T2, and the third sub-selection period T3, different video signals are input to the wirings 5621_1 to 5621_M. For example, a video signal input to the wiring 5621_J in the first sub-selection period Ti is input to the signal line Sj−1, a video signal input to the wiring 5621_J in the second sub-selection period T2 is input to the signal line Sj, and a video signal input to the wiring 5621_J in the third sub-selection period T3 is input to the signal line Sj+1. The video signals input to the wiring 5621_J in the first sub-selection period T1, the second sub-selection period T2, and the third sub-selection period T3 are denoted by Data_j−1, Data_j, and Data_j+1, respectively.

As shown in FIG. 13, in the first sub-selection period T1, the first thin film transistor 5603 a is on, and the second thin film transistor 5603 b and the third thin film transistor 5603 c are off. At this time, Data_j−1 input to the wiring 5621_J is input to the signal line Sj−1 through the first thin film transistor 5603 a. In the second sub-selection period T2, the second thin film transistor 5603 b is on, and the first thin film transistor 5603 a and the third thin film transistor 5603 c are off. At this time, Data_j input to the wiring 5621_J is input to the signal line Sj through the second thin film transistor 5603 b. In the third sub-selection period T3, the third thin film transistor 5603 c is on, and the first thin film transistor 5603 a and the second thin film transistor 5603 b are off. At this time, Data_j+1 input to the wiring 5621_J is input to the signal line Sj+1 through the third thin film transistor 5603 c.

As described above, in the signal line driver circuit of FIG. 12, one gate selection period is divided into three; thus, video signals can be input to three signal lines from one wiring 5621 in one gate selection period. Therefore, in the signal line driver circuit of FIG. 12, the number of connections between the substrate provided with the driver IC 5601 and the substrate provided with the pixel portion can be reduced to approximately one third of the number of signal lines. When the number of connections is reduced to approximately one third of the number of signal lines, the reliability, yield, and the like of the signal line driver circuit in FIG. 12 can be improved.

Note that there is no particular limitation on the arrangement, number, driving method, and the like of the thin film transistors, as long as one gate selection period is divided into a plurality of sub-selection periods and video signals are input to a plurality of signal lines from one wiring in each of the plurality of sub-selection periods as shown in FIG. 12.

For example, when video signals are input to three or more signal lines from one wiring in each of three or more sub-selection periods, a thin film transistor and a wiring for controlling the thin film transistor may be added. Note that when one gate selection period is divided into four or more sub-selection periods, one sub-selection period becomes short. Therefore, one gate selection period is preferably divided into two or three sub-selection periods.

As another example, as shown in a timing chart of FIG. 14, one selection period may be divided into a precharge period Tp, the first sub-selection period T1, the second sub-selection period T2, and the third sub-selection period T3. The timing chart of FIG. 14 shows timing when the scan line Gi in the i-th row is selected, timing 5803 a when the first thin film transistor 5603 a is turned on/off, timing 5803 b when the second thin film transistor 5603 b is turned on/off, timing 5803 c when the third thin film transistor 5603 c is turned on/off, and a signal 5821_J input to the wiring 5621_J in the J-th column. As shown in FIG. 14, the first thin film transistor 5603 a, the second thin film transistor 5603 b, and the third thin film transistor 5603 c are on in the precharge period Tp. At this time, precharge voltage Vp input to the wiring 5621_J is input to the signal line Sj−1, the signal line Sj, and the signal line Sj+1 through the first thin film transistor 5603 a, the second thin film transistor 5603 b, and the third thin film transistor 5603 c, respectively. In the first sub-selection period T1, the first thin film transistor 5603 a is on, and the second thin film transistor 5603 b and the third thin film transistor 5603 c are off. At this time, Data_j−1 input to the wiring 5621_J is input to the signal line Sj−1 through the first thin film transistor 5603 a. In the second sub-selection period T2, the second thin film transistor 5603 b is on, and the first thin film transistor 5603 a and the third thin film transistor 5603 c are off. At this time, Data_j input to the wiring 5621_J is input to the signal line Sj through the second thin film transistor 5603 b. In the third sub-selection period T3, the third thin film transistor 5603 c is on, and the first thin film transistor 5603 a and the second thin film transistor 5603 b are off. At this time, Data_j+1 input to the wiring 5621_J is input to the signal line Sj+1 through the third thin film transistor 5603 c.

As described above, in the signal line driver circuit of FIG. 12, to which the timing chart of FIG. 14 is applied, the signal line can be precharged by providing the precharge period before the sub-selection periods. Thus, a video signal can be written to a pixel with high speed. Note that portions in FIG. 14 which are similar to those in FIG. 13 are denoted by the same reference numerals, and detailed description of the same portions or portions having similar functions is omitted.

Now, a constitution of the scan line driver circuit is described. The scan line driver circuit includes a shift register and a buffer. Also, a level shifter may be included in some cases. In the scan line driver circuit, when a clock signal (CLK) and a start pulse signal (SP) are input to the shift register, a selection signal is produced. The generated selection signal is buffered and amplified by the buffer, and the resulting signal is supplied to a corresponding scan line. Gate electrodes of transistors in pixels corresponding to one line are connected to the scan line. Further, since the transistors in the pixels of one line have to be turned on at the same time, a buffer which can feed a large amount of current is used.

An example of a shift register used as part of the scan line driver circuit is described with reference to FIG. 15 and FIG. 16.

FIG. 15 illustrates a circuit configuration of the shift register. The shift register shown in FIG. 15 includes a plurality of flip-flops (flip-flops 5701_1 to 5701 _(—) n). Further, the shift register is operated by inputting a first clock signal, a second clock signal, a start pulse signal, and a reset signal.

Connection relationships of the shift register in FIG. 15 are described. In the flip-flop 5701 _(—) i (one of the flip-flops 5701_1 to 5701 _(—) n) of the i-th stage in the shift register of FIG. 15, a first wiring 5501 shown in FIG. 16 is connected to a seventh wiring 5717 _(—) i−1; a second wiring 5502 shown in FIG. 16 is connected to a seventh wiring 5717 _(—) i+1; a third wiring 5503 shown in FIG. 16 is connected to a seventh wiring 5717 _(—) i; and a sixth wiring 5506 shown in FIG. 16 is connected to a fifth wiring 5715.

Further, a fourth wiring 5504 shown in FIG. 16 is connected to a second wiring 5712 in flip-flops of odd-numbered stages, and is connected to a third wiring 5713 in flip-flops of even-numbered stages. A fifth wiring 5505 shown in FIG. 16 is connected to a fourth wiring 5714.

Note that the first wiring 5501 shown in FIG. 16 of the flip-flop 5701_1 of a first stage is connected to a first wiring 5711, and the second wiring 5502 shown in FIG. 16 of the flip-flop 5701 _(—) n of an n-th stage is connected to a sixth wiring 5716.

The first wiring 5711, the second wiring 5712, the third wiring 5713, and the sixth wiring 5716 may be referred to as a first signal line, a second signal line, a third signal line, and a fourth signal line, respectively. The fourth wiring 5714 and the fifth wiring 5715 may be referred to as a first power supply line and a second power supply line, respectively.

FIG. 16 illustrates the detail of the flip-flop shown in FIG. 15. A flip-flop shown in FIG. 16 includes a first thin film transistor 5571, a second thin film transistor 5572, a third thin film transistor 5573, a fourth thin film transistor 5574, a fifth thin film transistor 5575, a sixth thin film transistor 5576, a seventh thin film transistor 5577, and an eighth thin film transistor 5578. Note that the first thin film transistor 5571, the second thin film transistor 5572, the third thin film transistor 5573, the fourth thin film transistor 5574, the fifth thin film transistor 5575, the sixth thin film transistor 5576, the seventh thin film transistor 5577, and the eighth thin film transistor 5578 are n-channel transistors, and are brought into conduction when a voltage (V_(gs)) between a gate and a source exceeds a threshold voltage (V_(th)).

Now, a connection structure of the flip-flop shown in FIG. 16 is described below.

A first electrode (one of a source electrode or a drain electrode) of the first thin film transistor 5571 is connected to the fourth wiring 5504, and a second electrode (the other of the source electrode or the drain electrode) of the first thin film transistor 5571 is connected to the third wiring 5503.

A first electrode of the second thin film transistor 5572 is connected to the sixth wiring 5506. A second electrode of the second thin film transistor 5572 is connected to the third wiring 5503.

A first electrode of the third thin film transistor 5573 is connected to the fifth wiring 5505. A second electrode of the third thin film transistor 5573 is connected to a gate electrode of the second thin film transistor 5572. A gate electrode of the third thin film transistor 5573 is connected to the fifth wiring 5505.

A first electrode of the fourth thin film transistor 5574 is connected to the sixth wiring 5506. A second electrode of the fourth thin film transistor 5574 is connected to the gate electrode of the second thin film transistor 5572. A gate electrode of the fourth thin film transistor 5574 is connected to a gate electrode of the first thin film transistor 5571.

A first electrode of the fifth thin film transistor 5575 is connected to the fifth wiring 5505. A second electrode of the fifth thin film transistor 5575 is connected to the gate electrode of the first thin film transistor 5571. A gate electrode of the fifth thin film transistor 5575 is connected to the first wiring 5501.

A first electrode of the sixth thin film transistor 5576 is connected to the sixth wiring 5506. A second electrode of the sixth thin film transistor 5576 is connected to the gate electrode of the first thin film transistor 5571. A gate electrode of the sixth thin film transistor 5576 is connected to the gate electrode of the second thin film transistor 5572.

A first electrode of the seventh thin film transistor 5577 is connected to the sixth wiring 5506. A second electrode of the seventh thin film transistor 5577 is connected to the gate electrode of the first thin film transistor 5571. A gate electrode of the seventh thin film transistor 5577 is connected to the second wiring 5502. A first electrode of the eighth thin film transistor 5578 is connected to the sixth wiring 5506. A second electrode of the eighth thin film transistor 5578 is connected to the gate electrode of the second thin film transistor 5572. A gate electrode of the eighth thin film transistor 5578 is connected to the first wiring 5501.

Note that the point at which the gate electrode of the first thin film transistor 5571, the gate electrode of the fourth thin film transistor 5574, the second electrode of the fifth thin film transistor 5575, the second electrode of the sixth thin film transistor 5576, and the second electrode of the seventh thin film transistor 5577 are connected is referred to as a node 5543. The point at which the gate electrode of the second thin film transistor 5572, the second electrode of the third thin film transistor 5573, the second electrode of the fourth thin film transistor 5574, the gate electrode of the sixth thin film transistor 5576, and the second electrode of the eighth thin film transistor 5578 are connected is referred to as a node 5544.

The first wiring 5501, the second wiring 5502, the third wiring 5503, and the fourth wiring 5504 may be referred to as a first signal line, a second signal line, a third signal line, and a fourth signal line, respectively. The fifth wiring 5505 and the sixth wiring 5506 may be referred to as a first power supply line and a second power supply line, respectively.

Alternatively, the signal line driver circuit and the scan line driver circuit can be manufactured using only n-channel TFTs, which can be manufactured by a method similar to the method for manufacturing the non-linear element described in Embodiment 2 or 3. Since the n-channel TFTs which can be formed by a method similar to the method for manufacturing the non-linear element described in Embodiment 2 or 3 have high mobility, the driving frequency of the driver circuits can be increased. Further, the n-channel TFTs which can be formed by a method similar to the method for manufacturing the non-linear element described in Embodiment 2 or 3 include source regions or drain regions which are formed using an oxygen-deficiency oxide semiconductor layer including indium, gallium, and zinc. Therefore, the parasitic capacitance is decreased and the frequency characteristic (called f-characteristic) is increased. For example, the scan line driver circuit including the n-channel TFTs which can be formed by a method similar to the method for manufacturing the non-linear element described in Embodiment 2 or 3 can operate at high speed; therefore, it is possible to increase the frame frequency or to achieve insertion of a black screen, for example.

In addition, when the channel width of the transistor in the scan line driver circuit is increased or a plurality of scan line driver circuits is provided, for example, higher frame frequency can be realized. When a plurality of scan line driver circuits is provided, a scan line driver circuit for driving even-numbered scan lines is provided on one side and a scan line driver circuit for driving odd-numbered scan lines is provided on the opposite side; thus, increase in frame frequency can be realized.

In the case of manufacturing an active matrix type light-emitting display device, which is an example of a semiconductor device to which an embodiment of the present invention is applied, a plurality of scan line driver circuits is preferably arranged because a plurality of thin film transistors is arranged in at least one pixel. An example of a block diagram of an active matrix light-emitting display device is illustrated in FIG. 11B.

The light-emitting display device illustrated in FIG. 11B includes, over a substrate 5400, a pixel portion 5401 including a plurality of pixels each provided with a display element; a first scan line driver circuit 5402 and a second scan line driver circuit 5404 that select each pixel; and a signal line driver circuit 5403 that controls a video signal input to a selected pixel.

In the case of inputting a digital video signal to the pixel of the light-emitting display device of FIG. 11B, the pixel is put in a light-emitting state or non-light-emitting state by switching on/off of the transistor. Thus, grayscale can be displayed using an area ratio grayscale method or a time ratio grayscale method. An area ratio grayscale method refers to a driving method by which one pixel is divided into a plurality of subpixels and the respective subpixels are driven separately based on video signals so that grayscale is displayed. Further, a time ratio grayscale method refers to a driving method by which a period during which a pixel is in a light-emitting state is controlled so that grayscale is displayed.

Since the response time of light-emitting elements is shorter than that of liquid crystal elements or the like, the light-emitting elements are suitable for a time ratio grayscale method. Specifically, in the case of displaying by a time grayscale method, one frame period is divided into a plurality of subframe periods. Then, in accordance with video signals, the light-emitting element in the pixel is put in a light-emitting state or a non-light-emitting state in each subframe period. By dividing a frame into a plurality of subframes, the total length of time in which pixels actually emit light in one frame period can be controlled with video signals to display grayscales.

Note that in the light-emitting display device of FIG. 11B, in the case where one pixel includes a switching TFT and a current control TFT, a signal which is input to a first scan line serving as a gate wiring of the switching TFT is generated from the first scan line driver circuit 5402 and a signal which is input to a second scan line serving as a gate wiring of the current control TFT is generated from the second scan line driver circuit 5404. However, the signal which is input to the first scan line and the signal which is input to the second scan line may be generated together from one scan line driver circuit. In addition, for example, there is a possibility that a plurality of the first scan lines used for controlling the operation of the switching element be provided in each pixel depending on the number of transistors included in the switching element. In this case, the signals which are input to the first scan lines may be generated all from one scan line driver circuit or may be generated from a plurality of scan line driver circuits.

Even in the light-emitting display device, part of the driver circuit which can be formed using the n-channel TFTs can be provided over one substrate together with the thin film transistors of the pixel portion. Moreover, the signal line driver circuit and the scan line driver circuit can be manufactured using only the n-channel TFTs which can be formed by a method similar to the method for manufacturing the non-linear element described in Embodiment 2 or 3.

The aforementioned driver circuit may be used for not only a liquid crystal display device or a light-emitting display device but also electronic paper in which electronic ink is driven by utilizing an element electrically connected to a switching element. The electronic paper is also called an electrophoretic display device (electrophoretic display) and has advantages in that it has the same level of readability as regular paper, it has less power consumption than other display devices, and it can be set to have a thin and light form.

There are a variety of modes of electrophoretic displays. The electrophoretic display is a device in which a plurality of microcapsules each including first particles having positive charge and second particles having negative charge are dispersed in a solvent or a solute, and an electrical field is applied to the microcapsules so that the particles in the microcapsules move in opposite directions from each other, and only a color of the particles gathered on one side is displayed. Note that the first particles or the second particles include a colorant, and does not move when there is not electric field. Also, a color of the first particles is different from a color of the second particles (the particles may also be colorless).

Thus, the electrophoretic display utilizes a so-called dielectrophoretic effect, in which a substance with high dielectric constant moves to a region with high electric field. The electrophoretic display does not require a polarizing plate and a counter substrate, which are necessary for a liquid crystal display device, so that the thickness and weight thereof are about half.

That which the microcapsules are dispersed in a solvent is called electronic ink, and this electronic ink can be printed on a surface of glass, plastic, fabric, paper, or the like. Color display is also possible with the use of a color filter or particles including a coloring matter.

In addition, an active matrix type display device can be completed by providing as appropriate, a plurality of the microcapsules over an active matrix substrate so as to be interposed between two electrodes, and can perform display by application of electric field to the microcapsules. For example, the active matrix substrate obtained using the thin film transistors which can be formed by a method similar to the method for manufacturing the non-linear element described in Embodiment 2 or 3 can be used.

Note that the first particles and the second particles in the microcapsule may be formed from one of a conductive material, an insulating material, a semiconductor material, a magnetic material, a liquid crystal material, a ferroelectric material, an electroluminescent material, an electrochromic material, and a magnetophoretic material or a composite material thereof.

According to Embodiment 5, the protective circuit is formed using the non-linear element including an oxide semiconductor. Thus, a display device having a structure suitable as a protective circuit can be provided. In the connection structure between the first oxide semiconductor layer of the non-linear element and the wiring layers, the provision of the region which is bonded with the second oxide semiconductor layer, which has higher electrical conductivity than the first oxide semiconductor layer, allows stable operation as compared with the case of using only metal wirings. Accordingly, the function of the protective circuit is enhanced and the operation can be made stable. In this manner, according to Embodiment 5, a display device with high reliability can be manufactured. Further, by the structure similar to that of Embodiment 3, it is possible to manufacture a highly-reliable display device including a protective circuit including a non-linear element in which defects due to the peeling of the thin films are not easily caused.

Embodiment 5 can be combined with the structure disclosed in another Embodiment as appropriate.

Embodiment 6

A thin film transistor can be manufactured together with a non-linear element according to an embodiment of the present invention, and the thin film transistor can be used for a pixel portion and further for a driver circuit, so that a semiconductor device having a display function (also called a display device) can be manufactured. Moreover, a thin film transistor and a non-linear element according to an embodiment of the present invention can be used for part of a driver circuit or an entire driver circuit formed over one substrate together with a pixel portion, so that a system-on-panel can be formed.

The display device includes a display element. As the display element, a liquid crystal element (also referred to as a liquid crystal display element) or a light-emitting element (also referred to as a light-emitting display element) can be used. A light-emitting element includes, in its scope, an element whose luminance is controlled by current or voltage, and specifically includes an inorganic electroluminescent (EL) element, an organic EL element, and the like. Further, a display medium whose contrast is changed by an electric effect, such as electronic ink, can be used.

In addition, the display device includes a panel in which a display element is sealed, and a module in which an IC and the like including a controller are mounted on the panel. An embodiment of the present invention relates to one mode of an element substrate before the display element is completed in a process for manufacturing the display device, and the element substrate is provided with a means for supplying current to the display element in each of a plurality of pixels. Specifically, the element substrate may be in a state provided with only a pixel electrode of the display element, a state after a conductive film to be a pixel electrode is formed and before the conductive film is etched to form the pixel electrode, or any other states.

A display device in this specification refers to an image display device, a display device, or a light source (including a lighting device). Further, the display device includes any of the following modules in its category: a module including a connector such as an flexible printed circuit (FPC), a tape automated bonding (TAB) tape, or a tape carrier package (TCP); a module having a TAB tape or a TCP which is provided with a printed wiring board at the end thereof; and a module having an integrated circuit (IC) which is directly mounted on a display element by a chip-on-glass (COG) method.

The appearance and a cross section of a liquid crystal display panel, which is one mode of a display device according to an embodiment of the present invention, will be described in Embodiment 6 with reference to FIGS. 17A, 17B and 17C. FIGS. 17A and 17B are top views of a panel in which thin film transistors 4010 and 4011 with high electrical characteristics which can be manufactured by a method similar to the method for manufacturing the non-linear element, and a liquid crystal element 4013 are sealed with a sealant 4005 between a first substrate 4001 and a second substrate 4006. FIG. 17C corresponds to a cross section thereof along M-N of FIGS. 17A and 17B.

The sealant 4005 is provided so as to surround a pixel portion 4002 and a scan line driver circuit 4004 which are provided over the first substrate 4001. The second substrate 4006 is provided over the pixel portion 4002 and the scan line driver circuit 4004. Thus, the pixel portion 4002 and the scan line driver circuit 4004 as well as a liquid crystal layer 4008 are sealed with the sealant 4005 between the first substrate 4001 and the second substrate 4006. A signal line driver circuit 4003 that is formed using a single crystal semiconductor film or a polycrystalline semiconductor film over a substrate which is prepared separately is mounted in a region that is different from the region surrounded by the sealant 4005 over the first substrate 4001.

Note that there is no particular limitation on a connection method of the driver circuit which is separately formed, and a known COG method, wire bonding method, TAB method, or the like can be used. FIG. 17A illustrates an example in which the signal line driver circuit 4003 is mounted by a COG method and FIG. 17B illustrates an example in which signal line driver circuit 4003 is mounted by a TAB method.

Each of the pixel portion 4002 and the scan line driver circuit 4004 which are provided over the first substrate 4001 includes a plurality of thin film transistors. FIG. 17B illustrates the thin film transistor 4010 included in the pixel portion 4002 and the thin film transistor 4011 included in the scan line driver circuit 4004. Insulating layers 4020 and 4021 are provided over the thin film transistors 4010 and 4011.

Each of the thin film transistors 4010 and 4011 has high electrical characteristics and includes a gate insulating layer on which plasma treatment has been performed, a source region and a drain region which include an IGZO semiconductor film of oxygen-deficiency type, a source electrode layer and a drain electrode layer which are in contact with the source region and the drain region, and an IGZO semiconductor layer of oxygen-excess type which is in contact with the source region and the drain region. The thin film transistors 4010 and 4011 can be manufactured by a method similar to the method for manufacturing the non-linear element described in Embodiment 2. In Embodiment 6, the thin film transistors 4010 and 4011 are n-channel thin film transistors.

A pixel electrode layer 4030 included in the liquid crystal element 4013 is electrically connected to the thin film transistor 4010. A counter electrode layer 4031 of the liquid crystal element 4013 is formed on the second substrate 4006. A portion where the pixel electrode layer 4030, the counter electrode layer 4031, and the liquid crystal layer 4008 overlap with each other corresponds to the liquid crystal element 4013. Note that the pixel electrode layer 4030 and the counter electrode layer 4031 are provided with an insulating layer 4032 and an insulating layer 4033 serving as orientation films, respectively, and hold the liquid crystal layer 4008 with the insulating layers 4032 and 4033 interposed therebetween.

Note that the first substrate 4001 and the second substrate 4006 can be formed from glass, metal (typically, stainless steel), ceramic, or plastic. As plastic, a fiberglass-reinforced plastics (FRP) plate, a polyvinyl fluoride (PVF) film, a polyester film, or an acrylic resin film can be used. In addition, a sheet with a structure in which an aluminum foil is sandwiched between PVF films or polyester films can be used.

A columnar spacer 4035, which is formed by etching an insulating film selectively, is provided to control a distance (a cell gap) between the pixel electrode layer 4030 and the counter electrode layer 4031. Alternatively, a spherical spacer may be used.

Alternatively, a blue phase liquid crystal without an orientation film may be used. A blue phase is a type of liquid crystal phase, which appears just before a cholesteric liquid crystal changes into an isotropic phase when the temperature of the cholesteric liquid crystal is increased. A blue phase appears only within narrow temperature range; therefore, the liquid crystal layer 4008 is formed using a liquid crystal composition in which a chiral agent of 5 wt. % or more is mixed in order to expand the temperature range. The liquid crystal composition including a blue phase liquid crystal and a chiral agent has a short response time of 10 μs to 100 μs, and is optically isotropic; therefore, orientation treatment is not necessary and viewing angle dependence is small.

Note that Embodiment 6 describes an example of a transmissive liquid crystal display device; however, an embodiment of the present invention can be applied to a reflective liquid crystal display device or a semi-transmissive liquid crystal display device.

Although a liquid crystal display device of Embodiment 6 has a polarizer provided outer than the substrate (the viewer side) and a color layer and an electrode layer of a display element provided inner than the substrate, which are arranged in that order, the polarizer may be inner than the substrate. The stacked structure of the polarizer and the color layer is not limited to that shown in Embodiment 6 and may be set as appropriate in accordance with the materials of the polarizer and the color layer and the condition of the manufacturing process. Further, a light-blocking film serving as a black matrix may be provided.

In Embodiment 6, in order to reduce the unevenness of the surface of the thin film transistors and to improve the reliability of the thin film transistors, the non-linear element described in Embodiment 2 and the thin film transistors which can be formed by a method similar to the method for manufacturing the non-linear element are covered with protective films or insulating layers (the insulating layers 4020 and 4021) serving as planarizing insulating films. Note that the protective film is provided to prevent entry of a contaminant impurity such as an organic substance, a metal substance, or moisture floating in the atmosphere, and therefore a dense film is preferable. The protective film may be formed using a single layer or a stack of layers of a silicon oxide film, a silicon nitride film, a silicon oxynitride film, a silicon nitride oxide film, an aluminum oxide film, an aluminum nitride film, an aluminum oxynitride film, or an aluminum nitride oxide film by a sputtering method. Although the protective film is formed by a sputtering method in Embodiment 6, the method is not limited to a particular method and may be selected from a variety of methods.

Here, the insulating layer 4020 is formed to have a stacked structure as the protective film. Here, a silicon oxide film is formed by a sputtering method as a first layer of the insulating layer 4020. The use of a silicon oxide film for the protective film provides an advantageous effect of preventing hillock of an aluminum film used for a source electrode layer and a drain electrode layer.

Moreover, a silicon nitride film is formed by a sputtering method as a second layer of the insulating layer 4020. When a silicon nitride film is used for the protective film, it is possible to prevent movable ions such as sodium from entering a semiconductor region to vary the electrical characteristics of the TFT.

Further, after the protective film is formed, the IGZO semiconductor layer may be annealed (at 300° C. to 400° C.).

Further, the insulating layer 4021 is formed as the planarizing insulating film. The insulating layer 4021 can be formed from an organic material having heat resistance, such as polyimide, acrylic, benzocyclobutene, polyamide, or epoxy. As an alternative to such organic materials, it is possible to use a low-dielectric constant material (a low-k material), a siloxane-based resin, PSG (phosphosilicate glass), BPSG (borophosphosilicate glass), or the like. A siloxane-based resin may include as a substituent at least one of fluorine, an alkyl group, and an aryl group, as well as hydrogen. Note that the insulating layer 4021 may be formed by stacking a plurality of insulating films formed of these materials.

Note that a siloxane-based resin is a resin formed from a siloxane-based material as a starting material and having the bond of Si—O—Si. The siloxane-based resin may include as a substituent at least one of fluorine, an alkyl group, and aromatic hydrocarbon, as well as hydrogen.

The method for the formation of the insulating layer 4021 is not limited to a particular method and the following method can be used depending on the material of the insulating layer 4021: a sputtering method, an SOG method, spin coating, dip coating, spray coating, a droplet discharge method (e.g., an inkjet method, screen printing, or offset printing), a doctor knife, a roll coater, a curtain coater, a knife coater, or the like. In the case of forming the insulating layer 4021 with the use of a material solution, annealing (300° C. to 400° C.) may be performed on the IGZO semiconductor layer at the same time as a baking step. When the baking of the insulating layer 4021 and the annealing of the IGZO semiconductor layer are performed at the same time, a semiconductor device can be manufactured efficiently.

The pixel electrode layer 4030 and the counter electrode layer 4031 can be formed from a light-transmitting conductive material such as indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium tin oxide (hereinafter referred to as ITO), indium zinc oxide, or indium tin oxide to which silicon oxide is added.

A conductive composition including a conductive high molecule (also referred to as a conductive polymer) can be used for the pixel electrode layer 4030 and the counter electrode layer 4031. The pixel electrode formed of the conductive composition has preferably a sheet resistance of 10000 ohm/square or less and a transmittance of 70% or more at a wavelength of 550 nm. Further, the resistivity of the conductive high molecule included in the conductive composition is preferably 0.1 Ω·cm or less.

As the conductive high molecule, a so-called π-electron conjugated conductive polymer can be used. As examples thereof, polyaniline or a derivative thereof, polypyrrole or a derivative thereof, polythiophene or a derivative thereof, a copolymer of two or more kinds of them, and the like can be given.

Further, a variety of signals and potentials are supplied from an FPC 4018 to the signal line driver circuit 4003 which is formed separately, the scan line driver circuit 4004, and the pixel portion 4002.

In Embodiment 6, a connecting terminal electrode 4015 is formed using the same conductive film as the pixel electrode layer 4030 included in the liquid crystal element 4013. A terminal electrode 4016 is formed using the same conductive film as the source and drain electrode layers included in the thin film transistors 4010 and 4011.

The connecting terminal electrode 4015 is electrically connected to a terminal of the FPC 4018 through an anisotropic conductive film 4019.

Although FIGS. 17A, 17B and 17C show an example in which the signal line driver circuit 4003 is formed separately and mounted on the first substrate 4001, Embodiment 6 is not limited to this structure. The scan line driver circuit may be separately formed and then mounted, or only part of the signal line driver circuit or part of the scan line driver circuit may be separately formed and then mounted.

FIG. 18 illustrates an example in which a liquid crystal display module is formed as a semiconductor device using a TFT substrate 2600 manufactured according to an embodiment of the present invention.

FIG. 18 illustrates an example of a liquid crystal display module, in which the TFT substrate 2600 and a counter substrate 2601 are fixed to each other with a sealant 2602, and a pixel portion 2603 including a TFT and the like, a display element 2604 including a liquid crystal layer, and a color layer 2605 are provided between the substrates to form a display region. The color layer 2605 is necessary to perform color display. In the case of the RGB system, respective color layers corresponding to colors of red, green, and blue are provided for respective pixels. Polarizing plates 2606 and 2607 and a diffuser plate 2613 are provided outside the TFT substrate 2600 and the counter substrate 2601. A light source includes a cold cathode tube 2610 and a reflective plate 2611, and a circuit board 2612 is connected to a wiring circuit portion 2608 of the TFT substrate 2600 through a flexible wiring board 2609 and includes an external circuit such as a control circuit and a power source circuit. The polarizing plate and the liquid crystal layer may be stacked with a retardation plate interposed therebetween.

For the liquid crystal display module, a TN (Twisted Nematic) mode, an IPS (In-Plane-Switching) mode, an FFS (Fringe Field Switching) mode, an MVA (Multi-domain Vertical Alignment) mode, a PVA (Patterned Vertical Alignment) mode, an ASM (Axially Symmetric aligned Micro-cell) mode, an OCB (Optically Compensated Birefringence) mode, an FLC (Ferroelectric Liquid Crystal) mode, an AFLC (AntiFerroelectric Liquid Crystal) mode or the like can be used.

According to Embodiment 6, a protective circuit is formed using a non-linear element including an oxide semiconductor; thus, a display device having a structure suitable as a protective circuit can be provided. In the connection structure between the first oxide semiconductor layer of the non-linear element and the wiring layers, the provision of the region which is bonded with the second oxide semiconductor layer, which has higher electrical conductivity than the first oxide semiconductor layer, allows stable operation as compared with the case of using only metal wirings. Accordingly, the function of the protective circuit can be enhanced and the operation can be made stable. In this manner, according to Embodiment 6, a liquid crystal display panel with high reliability as a display device can be manufactured. Further, by the structure similar to that of Embodiment 3, it is possible to manufacture a highly-reliable liquid crystal display panel incorporating a protective circuit including a non-linear element in which defects due to the peeling of the thin films are not easily caused.

Embodiment 6 can be combined with the structure disclosed in another Embodiment as appropriate.

Embodiment 7

Embodiment 7 describes an example of a light-emitting display device as a display device according to an embodiment of the present invention. As an example of a display element of the display device, here, a light-emitting element utilizing electroluminescence is used. Light-emitting elements utilizing electroluminescence are classified according to whether a light emitting material is an organic compound or an inorganic compound. In general, the former is referred to as an organic EL element, the latter as an inorganic EL element.

In an organic EL element, by application of voltage to a light-emitting element, electrons and holes are separately injected from a pair of electrodes into a layer containing a light-emitting organic compound, and thus current flows. Then, those carriers (i.e., electrons and holes) are recombined, and thus, the light-emitting organic compound is excited. When the light-emitting organic compound returns to a ground state from the excited state, light is emitted. Owing to such a mechanism, such a light emitting element is referred to as a current-excitation light emitting element.

The inorganic EL elements are classified according to their element structures into a dispersion type inorganic EL element and a thin-film type inorganic EL element. A dispersion type inorganic EL element has a light-emitting layer where particles of a light-emitting material are dispersed in a binder, and its light emission mechanism is donor-acceptor recombination type light emission that utilizes a donor level and an acceptor level. A thin-film type inorganic EL element has a structure where a light-emitting layer is sandwiched between dielectric layers, which are further sandwiched between electrodes, and its light emission mechanism is localized type light emission that utilizes inner-shell electron transition of metal ions. Note that an organic EL element is used as a light-emitting element in this example.

FIG. 19 illustrates an example of a pixel structure to which digital time grayscale driving can be applied, as an example of a semiconductor device to which an embodiment of the present invention is applied.

A structure and operation of a pixel to which digital time grayscale driving can be applied are described. In this example, one pixel includes two n-channel transistors in each of which a channel formation region includes an IGZO semiconductor layer and which can be formed by a method similar to the method for manufacturing the non-linear element described in Embodiment 2 or 3.

A pixel 6400 includes a switching transistor 6401, a driver transistor 6402, a light-emitting element 6404, and a capacitor 6403. A gate of the switching transistor 6401 is connected to a scan line 6406, a first electrode (one of a source electrode and a drain electrode) of the switching transistor 6401 is connected to a signal line 6405, and a second electrode (the other of the source electrode and the drain electrode) of the switching transistor 6401 is connected to a gate of the driver transistor 6402. The gate of the driver transistor 6402 is connected to a power supply line 6407 through the capacitor 6403, a first electrode of the driver transistor 6402 is connected to the power supply line 6407, and a second electrode of the driver transistor 6402 is connected to a first electrode (pixel electrode) of the light-emitting element 6404. A second electrode of the light-emitting element 6404 corresponds to a common electrode 6408.

The second electrode (common electrode 6408) of the light-emitting element 6404 is set to a low power supply potential. The low power supply potential is a potential satisfying the low power supply potential < a high power supply potential when the high power supply potential set to the power supply line 6407 is a reference. As the low power supply potential, GND, 0 V, or the like may be employed, for example. A potential difference between the high power supply potential and the low power supply potential is applied to the light-emitting element 6404 and current is supplied to the light-emitting element 6404, so that the light-emitting element 6404 emits light. Here, in order to make the light-emitting element 6404 emit light, each potential is set so that the potential difference between the high power supply potential and the low power supply potential is greater than or equal to forward threshold voltage.

Gate capacitance of the driver transistor 6402 may be used as a substitute for the capacitor 6403, so that the capacitor 6403 can be omitted. The gate capacitance of the driver transistor 6402 may be formed between the channel region and the gate electrode.

In the case of a voltage-input voltage driving method, a video signal is input to the gate of the driver transistor 6402 so that the driver transistor 6402 is in either of two states of being sufficiently turned on and turned off. That is, the driver transistor 6402 operates in a linear region. Since the driver transistor 6402 operates in a linear region, a voltage higher than the voltage of the power supply line 6407 is applied to the gate of the driver transistor 6402. Note that a voltage higher than or equal to (voltage of the power supply line +Vth of the driver transistor 6402) is applied to the signal line 6405.

In the case of performing analog grayscale driving instead of digital time grayscale driving, the same pixel structure as that in FIG. 19 can be used by changing signal input.

In the case of performing analog grayscale driving, a voltage higher than or equal to (forward voltage of the light-emitting element 6404+Vth of the driver transistor 6402) is applied to the gate of the driver transistor 6402. The forward voltage of the light-emitting element 6404 indicates a voltage at which a desired luminance is obtained, and includes at least forward threshold voltage. The video signal by which the driver transistor 6402 operates in a saturation region is input, so that current can be supplied to the light-emitting element 6404. In order for the driver transistor 6402 to operate in a saturation region, the potential of the power supply line 6407 is set higher than the gate potential of the driver transistor 6402. When an analog video signal is used, it is possible to feed current to the light-emitting element 6404 in accordance with the video signal and perform analog grayscale driving.

The pixel structure shown in FIG. 19 is not limited thereto. For example, a switch, a resistor, a capacitor, a transistor, a logic circuit, or the like may be added to the pixel shown in FIG. 19.

Next, structures of a light-emitting element are described with reference to FIGS. 20A to 20C. A cross-sectional structure of a pixel is described here by taking an n-channel driver TFT as an example. TFTs 7001, 7011, and 7021 serving as driver TFTs used for a semiconductor device, which are illustrated in FIGS. 20A, 20B, and 20C, can be formed by a method similar to the method for manufacturing the non-linear element described in Embodiment 2. The TFTs 7001, 7011, and 7021 have high electrical characteristics and each include a gate insulating layer on which plasma treatment has been performed, a source region and a drain region which include an IGZO semiconductor film of oxygen-deficiency type, a source electrode layer and a drain electrode layer which are in contact with the source region and the drain region, and an IGZO semiconductor layer of oxygen-excess type which is in contact with the source region and the drain region.

In addition, in order to extract light emitted from the light-emitting element, at least one of an anode and a cathode is required to transmit light. A thin film transistor and a light-emitting element are formed over a substrate. A light-emitting element can have a top-emission structure in which light emission is extracted through the surface opposite to the substrate; a bottom-emission structure in which light emission is extracted through the surface on the substrate side; or a dual-emission structure in which light emission is extracted through the surface opposite to the substrate and the surface on the substrate side. The pixel structure according to an embodiment of the present invention can be applied to a light-emitting element having any of these emission structures.

A light-emitting element with a top-emission structure is described with reference to FIG. 20A.

FIG. 20A is a cross-sectional view of a pixel in a case where the driver TFT 7001 is an n-channel TFT and light generated in a light-emitting element 7002 is emitted to an anode 7005 side with respect to a light-emitting layer 7004 (the side opposite to the substrate side). In FIG. 20A, a cathode 7003 of the light-emitting element 7002 is electrically connected to the driver TFT 7001, and the light-emitting layer 7004 and the anode 7005 are stacked in this order over the cathode 7003. The cathode 7003 can be formed using any of a variety of conductive materials as long as it has a low work function and reflects light. For example, Ca, Al, CaF, MgAg, AlLi, or the like is preferably used. The light-emitting layer 7004 may be formed using a single layer or by stacking a plurality of layers. When the light-emitting layer 7004 is formed using a plurality of layers, the light-emitting layer 7004 is formed by stacking an electron-injecting layer, an electron-transporting layer, a light-emitting layer, a hole-transporting layer, and a hole-injecting layer in this order over the cathode 7003. It is not necessary to form all of these layers. The anode 7005 is formed using a light-transmitting conductive film such as a film of indium oxide including tungsten oxide, indium zinc oxide including tungsten oxide, indium oxide including titanium oxide, indium tin oxide including titanium oxide, indium tin oxide (hereinafter, referred to as ITO), indium zinc oxide, or indium tin oxide to which silicon oxide is added.

The light-emitting element 7002 corresponds to a region where the cathode 7003 and the anode 7005 sandwich the light-emitting layer 7004. In the case of the pixel shown in FIG. 20A, light is emitted from the light-emitting element 7002 to the anode 7005 side as indicated by an arrow.

Next, a light-emitting element having the bottom-emission structure is described with reference to FIG. 20B. FIG. 20B is a cross-sectional view of a pixel in the case where a driver TFT 7011 is n-channel, and light is emitted from a light-emitting element 7012 to a cathode 7013 side with respect to a light-emitting layer 7014 (the substrate side). In FIG. 20B, the cathode 7013 of the light-emitting element 7012 is formed over a light-transmitting conductive film 7017 which is electrically connected to the driver TFT 7011, and the light-emitting layer 7014 and an anode 7015 are stacked in this order over the cathode 7013. A light-blocking film 7016 for reflecting or blocking light may be formed so as to cover the anode 7015 when the anode 7015 has a light-transmitting property. For the cathode 7013, a variety of materials can be used as in the case of FIG. 20A as long as the cathode 7013 is a conductive film having a low work function. Note that the cathode 7013 is formed to have a thickness that can transmit light (preferably, approximately from 5 nm to 30 nm). For example, an aluminum film with a thickness of 20 nm can be used as the cathode 7013. The light-emitting layer 7014 may be formed of a single layer or by stacking a plurality of layers as in the case of FIG. 20A. The anode 7015 is not required to transmit light, but can be formed using a light-transmitting conductive material as in the case of FIG. 20A. For the light-blocking film 7016, metal or the like that reflects light can be used; however, it is not limited to a metal film. For example, a resin or the like to which black pigment is added can be used.

The light-emitting element 7012 corresponds to a region where the cathode 7013 and the anode 7015 sandwich the light-emitting layer 7014. In the case of the pixel shown in FIG. 20B, light is emitted from the light-emitting element 7012 to the cathode 7013 side as indicated by an arrow.

Next, a light-emitting element having a dual-emission structure is described with reference to FIG. 20C. In FIG. 20C, a cathode 7023 of a light-emitting element 7022 is formed over a light-transmitting conductive film 7027 which is electrically connected to the driver TFT 7021, and a light-emitting layer 7024 and an anode 7025 are stacked in this order over the cathode 7023. As in the case of FIG. 20A, the cathode 7023 can be formed of any of a variety of conductive materials as long as it is conductive and has low work function. Note that the cathode 7023 is formed to have a thickness that can transmit light. For example, an Al film having a thickness of 20 nm can be used as the cathode 7023. The light-emitting layer 7024 may be formed using a single layer or by stacking a plurality of layers as in the case of FIG. 20A. In a manner similar to FIG. 20A, the anode 7025 can be formed using a light-transmitting conductive material.

The light-emitting element 7022 corresponds to a region where the cathode 7023, the light-emitting layer 7024, and the anode 7025 overlap with each other. In the pixel illustrated in FIG. 20C, light is emitted from the light-emitting element 7022 to both the anode 7025 side and the cathode 7013 side as denoted by arrows.

Although an organic EL element is described here as a light-emitting element, an inorganic EL element can be alternatively provided as a light-emitting element.

Note that Embodiment 7 describes the example in which a thin film transistor (driver TFT) which controls the driving of a light-emitting element is electrically connected to the light-emitting element, but a structure may be employed in which a current control TFT is connected between the driver TFT and the light-emitting element.

The semiconductor device described in Embodiment 7 is not limited to the structures illustrated in FIGS. 20A to 20C, and can be modified in various ways based on the spirit of techniques according to the present invention.

Next, the appearance and cross section of a light-emitting display panel (also referred to as a light-emitting panel) which corresponds to one mode of a semiconductor device according to the present invention will be described with reference to FIGS. 21A and 21B. FIG. 21A is a top view of a panel in which a light-emitting element and a thin film transistor having high electrical characteristics that can be manufactured over a first substrate by a method similar to the method for manufacturing a non-linear element according to an embodiment of the present invention is sealed between the first substrate and a second substrate with a sealant, and FIG. 21B is a cross-sectional view along H—I of FIG. 21A.

A sealant 4505 is provided so as to surround a pixel portion 4502, signal line driver circuits 4503 a and 4503 b, and scan line driver circuits 4504 a and 4504 b, which are provided over a first substrate 4501. In addition, a second substrate 4506 is formed over the pixel portion 4502, the signal line driver circuits 4503 a and 4503 b, and the scan line driver circuits 4504 a and 4504 b. Accordingly, the pixel portion 4502, the signal line driver circuits 4503 a and 4503 b, and the scan line driver circuits 4504 a and 4504 b are sealed, together with filler 4507, with the first substrate 4501, the sealant 4505, and the second substrate 4506. In this manner, it is preferable that the pixel portion 4502, the signal line driver circuits 4503 a and 4503 b, and the scan line driver circuits 4504 a and 4504 b be packaged (sealed) with a protective film (such as an attachment film or an ultraviolet curable resin film) or a cover material with high air-tightness and little degasification so that the pixel portion 4502, the signal line driver circuits 4503 a and 4503 b, and the scan line driver circuits 4504 a and 4504 b are not exposed to external air.

The pixel portion 4502, the signal line driver circuits 4503 a and 4503 b, and the scan line driver circuits 4504 a and 4504 b formed over the first substrate 4501 each include a plurality of thin film transistors, and the thin film transistor 4510 included in the pixel portion 4502 and the thin film transistor 4509 included in the signal line driver circuit 4503 a are illustrated as an example in FIG. 21B.

Each of the thin film transistors 4509 and 4510 has high electrical characteristics and includes a gate insulating layer on which plasma treatment has been performed, a source region and a drain region which are formed using an IGZO semiconductor film of oxygen-deficiency type, a source electrode layer and a drain electrode layer which are in contact with the source region and the drain region, and an IGZO semiconductor layer of oxygen-excess type which is in contact with the source region and the drain region. The thin film transistors 4509 and 4510 can be manufactured in a manner similar to the method for manufacturing the non-linear element described in Embodiment 2. In Embodiment 7, the thin film transistors 4509 and 4510 are n-channel thin film transistors.

Moreover, reference numeral 4511 denotes a light-emitting element. A first electrode layer 4517 which is a pixel electrode included in the light-emitting element 4511 is electrically connected to source and drain electrode layers of the thin film transistor 4510. Note that although the light-emitting element 4511 has a stacked structure of the first electrode layer 4517, an electroluminescent light-emitting layer 4512, and a second electrode layer 4513, the structure of the light-emitting element 4511 is not limited to the structure shown in Embodiment 7. The structure of the light-emitting element 4511 can be changed as appropriate depending on a direction in which light is extracted from the light-emitting element 4511, or the like.

A partition wall 4520 is formed using an organic resin film, an inorganic insulating film, or organic polysiloxane. It is particularly preferable that the partition wall 4520 be formed using a photosensitive material to have an opening portion on the first electrode layer 4517 so that a sidewall of the opening portion is formed as a tilted surface with continuous curvature.

The electroluminescent light-emitting layer 4512 may be formed using a single layer or a plurality of layers stacked.

In order to prevent entry of oxygen, hydrogen, moisture, carbon dioxide, or the like into the light-emitting element 4511, a protective film may be formed over the second electrode layer 4513 and the partition wall 4520. As the protective film, a silicon nitride film, a silicon nitride oxide film, a DLC (diamond like carbon) film, or the like can be formed.

In addition, a variety of signals and potentials are supplied from FPCs 4518 a and 4518 b to the signal line driver circuits 4503 a and 4503 b, the scan line driver circuits 4504 a and 4504 b, or the pixel portion 4502.

In Embodiment 7, a connecting terminal electrode 4515 is formed using the same conductive film as the first electrode layer 4517 included in the light-emitting element 4511. A terminal electrode 4516 is formed using the same conductive film as the source and drain electrode layers included in the thin film transistors 4509 and 4510.

The connecting terminal electrode 4515 is electrically connected to a terminal included in the FPC 4518 a through an anisotropic conductive film 4519.

The second substrate 4506 located in the direction in which light is extracted from the light-emitting element 4511 needs to have a light-transmitting property. In that case, a light-transmitting material such as a glass plate, a plastic plate, a polyester film, or an acrylic film is used.

As the filler 4507, an ultraviolet curable resin or a thermosetting resin as well as inert gas such as nitrogen or argon can be used. For example, polyvinyl chloride (PVC), acrylic, polyimide, an epoxy resin, a silicone resin, polyvinyl butyral (PVB), or ethylene vinyl acetate (EVA) can be used. In Embodiment 7, nitrogen is used for the filler 4507.

In addition, if needed, optical films such as a polarizing plate, a circularly polarizing plate (including an elliptically polarizing plate), a retarder plate (a quarter-wave plate, a half-wave plate), and a color filter may be provided on an emission surface of the light-emitting element, as appropriate. Further, the polarizing plate or the circularly polarizing plate may be provided with an anti-reflection film. For example, anti-glare treatment can be performed by which reflected light is diffused in the depression/projection of the surface and glare can be reduced.

As the signal line driver circuits 4503 a and 4503 b and the scan line driver circuits 4504 a and 4504 b, driver circuits formed by using a single crystal semiconductor film or polycrystalline semiconductor film over a substrate separately prepared may be mounted. In addition, only the signal line driver circuit or only part thereof, or only the scan line driver circuit or only part thereof may be separately formed to be mounted. Embodiment 7 is not limited to the structure shown in FIGS. 21A and 21B.

According to Embodiment 7, a protective circuit is formed using a non-linear element including an oxide semiconductor; thus, a display device having a structure suitable as a protective circuit can be provided. In the connection structure between the first oxide semiconductor layer of the non-linear element and the wiring layers, the provision of the region which is bonded with the second oxide semiconductor layer, which has higher electrical conductivity than the first oxide semiconductor layer, allows stable operation as compared with the case of using only metal wirings. Accordingly, the function of the protective circuit is enhanced and the operation can be made stable. In this manner, according to Embodiment 7, a light-emitting display device (display panel) with high reliability as a display device can be manufactured. Further, by the structure similar to that of Embodiment 3, it is possible to manufacture a highly-reliable light-emitting display device (display panel) including a protective circuit including a non-linear element in which defects due to the peeling of the thin films are not easily caused.

Embodiment 7 can be combined with the structure disclosed in another Embodiment as appropriate.

Embodiment 8

A display device according to an embodiment of the present invention can be applied as electronic paper. Electronic paper can be used for electronic appliances of every field for displaying information. For example, electronic paper can be used for electronic book (e-book), posters, advertisement in vehicles such as trains, display in a variety of cards such as credit cards, and so on. Examples of such electronic appliances are illustrated in FIGS. 22A and 22B and FIG. 23.

FIG. 22A illustrates a poster 2631 formed using electronic paper. If the advertizing medium is printed paper, the advertisement is replaced by manpower; however, when electronic paper to which an embodiment of the present invention is applied is used, the advertisement display can be changed in a short time. Moreover, a stable image can be obtained without display deterioration. Further, the poster may send and receive information wirelessly.

FIG. 22B illustrates an advertisement 2632 in a vehicle such as a train. If the advertizing medium is printed paper, the advertisement is replaced by manpower; however, when electronic paper to which an embodiment of the present invention is applied is used, the advertisement display can be changed in a short time without much manpower. Moreover, a stable image can be obtained without display deterioration. Further, the advertisement in vehicles may send and receive information wirelessly.

FIG. 23 illustrates an example of an electronic book device 2700. For example, the electronic book device 2700 includes two housings 2701 and 2703. The housings 2701 and 2703 are bound with each other by an axis portion 2711, along which the electronic book device 2700 is opened and closed. With such a structure, operation as a paper book is achieved.

A display portion 2705 is incorporated in the housing 2701 and a display portion 2707 is incorporated in the housing 2703. The display portion 2705 and the display portion 2707 may display a series of images, or may display different images. In the structure where different images are displayed in different display portions, for example, the right display portion (the display portion 2705 in FIG. 23) can display text and the left display portion (the display portion 2707 in FIG. 23) can display images.

FIG. 23 illustrates an example in which the housing 2701 is provided with an operation portion and the like. For example, the housing 2701 is provided with a power supply 2721, an operation key 2723, a speaker 2725, and the like. The page can be turned with the operation key 2723. Note that a keyboard, a pointing device, and the like may be provided on the same plane as the display portion of the housing. Further, a rear surface or a side surface of the housing may be provided with an external connection terminal (an earphone terminal, a USB terminal, a terminal which can be connected with a variety of cables such as an AC adopter or a USB cable, and the like), a storage medium inserting portion, or the like. Moreover, the electronic book device 2700 may have a function of an electronic dictionary.

Further, the electronic book device 2700 may send and receive information wirelessly. Desired book data or the like can be purchased and downloaded from an electronic book server wirelessly.

As described in Embodiment 8, an electronic appliance with high reliability can be provided when a display device having a protective circuit whose function has been enhanced by the use of a non-linear element including an oxide semiconductor and whose operation has been made stable is mounted on the electronic appliance. By the use of a structure similar to that of Embodiment 3, it is possible to manufacture an electronic appliance including a display device with high reliability, on which a protective circuit including a non-linear element in which a defect due to peeling of a thin film does not easily occur is mounted.

Embodiment 8 can be combined with the structure disclosed in another Embodiment as appropriate.

Embodiment 9

A semiconductor device according to an embodiment of the present invention can be applied to a variety of electronic appliances (including game machines). As the electronic appliances, for example, there are a television device (also called TV or a television receiver), a monitor for a computer or the like, a digital camera, a digital video camera, a digital photo frame, a cellular phone (also called a mobile phone or a portable telephone device), a portable game machine, a portable information terminal, an audio playback device, and a large game machine such as a pachinko machine.

FIG. 24A illustrates an example of a television device 9600. A display portion 9603 is incorporated in a housing 9601 of the television device 9600. The display portion 9603 can display images. Here, the housing 9601 is supported on a stand 9605.

The television device 9600 can be operated by an operation switch of the housing 9601 or a separate remote controller 9610. The channel and volume can be controlled with operation keys 9609 of the remote controller 9610 and the images displayed in the display portion 9603 can be controlled. Moreover, the remote controller 9610 may have a display portion 9607 in which the information outgoing from the remote controller 9610 is displayed.

Note that the television device 9600 is provided with a receiver, a modem, and the like. With the use of the receiver, general television broadcasting can be received. Moreover, when the display device is connected to a communication network with or without wires via the modem, one-way (from a sender to a receiver) or two-way (between a sender and a receiver or between receivers) information communication can be performed.

FIG. 24B illustrates an example of a digital photo frame 9700. For example, a display portion 9703 is incorporated in a housing 9701 of the digital photo frame 9700. The display portion 9703 can display a variety of images, for example, displays image data taken with a digital camera or the like, so that the digital photo frame can function in a manner similar to a general picture frame.

Note that the digital photo frame 9700 is provided with an operation portion, an external connection terminal (such as a USB terminal or a terminal which can be connected to a variety of cables including a USB cable), a storage medium inserting portion, and the like. These structures may be incorporated on the same plane as the display portion; however, they are preferably provided on the side surface or rear surface of the display portion because the design is improved. For example, a memory including image data taken with a digital camera is inserted into the storage medium inserting portion of the digital photo frame and the image data is imported. Then, the imported image data can be displayed in the display portion 9703.

The digital photo frame 9700 may send and receive information wirelessly. In this case, desired image data can be wirelessly imported into the digital photo frame 9700 and can be displayed therein.

FIG. 25A illustrates a portable game machine including a housing 9881 and a housing 9891 which are jointed with a connector 9893 so as to be able to open and close. A display portion 9882 and a display portion 9883 are incorporated in the housing 9881 and the housing 9891, respectively. The portable game machine illustrated in FIG. 25A additionally includes a speaker portion 9884, a storage medium inserting portion 9886, an LED lamp 9890, an input means (operation keys 9885, a connection terminal 9887, a sensor 9888 (including a function of measuring force, displacement, position, speed, acceleration, angular speed, the number of rotations, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, electric power, radiation, flow rate, humidity, tilt angle, vibration, smell, or infrared ray), a microphone 9889, and the like). Needless to say, the structure of the portable game machine is not limited to the above, and may be any structure as long as a semiconductor device according to an embodiment of the present invention is provided. Moreover, another accessory may be provided as appropriate. The portable game machine shown in FIG. 25A has a function of reading out a program or data stored in a storage medium to display it on the display portion, and a function of sharing information with another portable game machine by wireless communication. The portable game machine in FIG. 25A can have a variety of functions other than those above.

FIG. 25B illustrates an example of a slot machine 9900, which is a large game machine. A display portion 9903 is incorporated in a housing 9901 of the slot machine 9900. The slot machine 9900 additionally includes an operation means such as a start lever or a stop switch, a coin slot, a speaker, and the like. Needless to say, the structure of the slot machine 9900 is not limited to the above, and may be any structure as long as at least a semiconductor device according to an embodiment of the present invention is provided. Moreover, another accessory may be provided as appropriate.

FIG. 26 illustrates an example of a cellular phone 1000. The cellular phone 1000 includes a housing 1001 in which a display portion 1002 is incorporated, and moreover includes an operation button 1003, an external connection port 1004, a speaker 1005, a microphone 1006, and the like.

Information can be input to the cellular phone 1000 illustrated in FIG. 26 by touching the display portion 1002 with a finger or the like. Moreover, making a call or text messaging can be performed by touching the display portion 1002 with a finger or the like.

There are mainly three screen modes of the display portion 1002. The first mode is a display mode mainly for displaying an image. The second mode is an input mode mainly for inputting information such as text. The third mode is a display-and-input mode in which two modes of the display mode and the input mode are mixed.

For example, in the case of making a call or text messaging, the display portion 1002 is set to a text input mode where text input is mainly performed, and text input operation can be performed on a screen. In this case, it is preferable to display a keyboard or number buttons on almost the entire screen of the display portion 1002.

When a detection device including a sensor for detecting inclination, such as a gyroscope or an acceleration sensor, is provided inside the cellular phone 1000, display in the screen of the display portion 1002 can be automatically switched by judging the direction of the cellular phone 1000 (whether the cellular phone 1000 is placed horizontally or vertically for a landscape mode or a portrait mode).

Further, the screen modes are switched by touching the display portion 1002 or operating the operation button 1003 of the housing 1001. Alternatively, the screen modes can be switched depending on kinds of images displayed in the display portion 1002. For example, when a signal for an image displayed in the display portion is data of moving images, the screen mode is switched to the display mode. When the signal is text data, the screen mode is switched to the input mode.

Moreover, in the input mode, when input by touching the display portion 1002 is not performed within a specified period while a signal detected by an optical sensor in the display portion 1002 is detected, the screen mode may be controlled so as to be switched from the input mode to the display mode.

The display portion 1002 can also function as an image sensor. For example, an image of a palm print, a fingerprint, or the like is taken by touching the display portion 1002 with the palm or the finger, whereby personal authentication can be performed. Moreover, when a backlight which emits near-infrared light or a sensing light source which emits near-infrared light is provided in the display portion, a finger vein, a palm vein, or the like can be taken.

As described in Embodiment 9, an electronic appliance with high reliability can be provided when a display device having a protective circuit whose function has been improved by the use of a non-linear element including an oxide semiconductor and whose operation has been made stable is mounted on the electronic appliance. By the use of a structure similar to that of Embodiment 3, it is possible to manufacture an electronic appliance including a display device with high reliability, on which a protective circuit including a non-linear element in which a defect due to peeling of a thin film does not easily occur is mounted.

Embodiment 9 can be combined with the structure disclosed in another Embodiment as appropriate.

This application is based on Japanese Patent Application serial No. 2008-235581 filed with Japan Patent Office on Sep. 12, 2008, the entire contents of which are hereby incorporated by reference.

EXPLANATION OF REFERENCE

-   10: substrate, 11: scan line input terminal, 12: signal line input     terminal, 13: scan line, 14: signal line, 15: gate electrode, 16:     gate electrode, 17: pixel portion, 18: pixel, 19: pixel transistor,     20: storage capacitor portion, 21: pixel electrode, 22: capacitor     line, 23: common terminal, 24: protective circuit, 25: protective     circuit, 26: protective circuit, 27: capacitor bus line, 28: common     wiring, 29: common wiring, 30: non-linear element, 30 a: non-linear     element, 30 b: non-linear element, 31: non-linear element, 31 a:     non-linear element, 31 b: non-linear element, 36: oxide     semiconductor layer, 37: gate insulating layer, 38: wiring layer,     39: wiring layer, 40: oxide semiconductor layer, 41: conductive     layer, 42: interlayer insulating layer, 43: contact hole, 44: wiring     layer, 100: substrate, 101: gate electrode, 102: gate insulating     layer, 103: oxide semiconductor layer, 104 a: source region, 104 b:     drain region, 105 a: source electrode layer, 105 b: drain electrode     layer, 106 a: source region, 106 b: drain region, 107: protective     insulating film, 108: scan line, 111 a: oxide semiconductor layer,     111 b: oxide semiconductor layer, 125: contact hole, 126: contact     hole, 128: wiring layer, 580: substrate, 581: thin film transistor,     583: interlayer insulating layer, 584: protective film, 585:     insulating layer, 587: electrode layer, 588: electrode layer, 589:     spherical particle, 590 a: black region, 590 b: white region, 594:     cavity, 595: filler, 596: substrate, 650: scan line, 651: common     wiring, 730 a: non-linear element, 730 b: non-linear element, 730 c:     non-linear element, 740 a: non-linear element, 740 b: non-linear     element, 740 c: non-linear element, 740 d: non-linear element, 1000:     cellular phone, 1001: housing, 1002: display portion, 1003:     operation button, 1004: external connection port, 1005: speaker,     1006: microphone, 2600: TFT substrate, 2601: counter substrate,     2602: sealant, 2603: pixel portion, 2604: display element, 2605:     color layer, 2606: polarizing plate, 2607: polarizing plate, 2608:     wiring circuit portion, 2609: flexible wiring board, 2610: cold     cathode tube, 2611: reflecting plate, 2612: circuit board, 2613:     diffusion plate, 2631: poster, 2632: advertisement in vehicle, 2700:     electronic book device, 2701: housing, 2703: housing, 2705: display     portion, 2707: display portion, 2711: axis portion, 2721: power     supply, 2723: operation key, 2725: speaker, 4001: substrate, 4002:     pixel portion, 4003: signal line driver circuit, 4004: scan line     driver circuit, 4005: sealant, 4006: substrate, 4008: liquid crystal     layer, 4010: thin film transistor, 4011: thin film transistor, 4013:     liquid crystal element, 4015: connecting terminal electrode, 4016:     terminal electrode, 4018: FPC, 4019: anisotropic conductive film,     4020: insulating layer, 4021: insulating layer, 4030: pixel     electrode layer, 4031: counter electrode layer, 4032: insulating     layer, 4033: insulating layer, 4035: spacer, 4501: substrate, 4502:     pixel portion, 4503 a: signal line driver circuit, 4503 b: signal     line driver circuit, 4504 a: scan line driver circuit, 4504 b: scan     line driver circuit, 4505: sealing material, 4506: substrate, 4507:     filler, 4509: thin film transistor, 4510: thin film transistor,     4511: light-emitting element, 4512: electric field light-emitting     layer, 4513: electrode layer, 4515: connecting terminal electrode,     4516: terminal electrode, 4517: electrode layer, 4518 a: FPC, 4518     b: FPC, 4519: anisotropic conductive film, 4520: partition wall,     5300: substrate, 5301: pixel portion, 5302: scan line driver     circuit, 5303: signal line driver circuit, 5400: substrate, 5401:     pixel portion, 5402: scan line driver circuit, 5403: signal line     driver circuit, 5404: scan line driver circuit, 5501: wiring, 5502:     wiring, 5503: wiring, 5504: wiring, 5505: wiring, 5506: wiring,     5543: node, 5544: node, 5571: thin film transistor, 5572: thin film     transistor, 5573: thin film transistor, 5574: thin film transistor,     5575: thin film transistor, 5576: thin film transistor, 5577: thin     film transistor, 5578: thin film transistor, 5601: driver IC, 5602:     switch group, 5603 a: thin film transistor, 5603 b: thin film     transistor, 5603 c: thin film transistor, 5611: wiring, 5612:     wiring, 5613: wiring, 5621: wiring, 5701: flip-flop, 5703 a: timing,     5703 b: timing, 5703 c: timing, 5711: wiring, 5712: wiring, 5713:     wiring, 5714: wiring, 5715: wiring, 5716: wiring, 5717: wiring,     5721: signal, 5803 a: timing, 5803 b: timing, 5803 c: timing, 5821:     signal, 6400: pixel, 6401: switching transistor, 6402: driver     transistor, 6403: capacitor, 6404: light-emitting element, 6405:     signal line, 6406: scan line, 6407: power supply line, 6408: common     electrode, 7001: driver TFT, 7002: light-emitting element, 7003:     cathode, 7004: light-emitting layer, 7005: anode, 7011: driver TFT,     7012: light-emitting element, 7013: cathode, 7014: light-emitting     layer, 7015: anode, 7016: light-blocking film, 7017: conductive     film, 7021: driver TFT, 7022: light-emitting element, 7023: cathode,     7024: light-emitting layer, 7025: anode, 7027: conductive film,     9600: television device, 9601: housing, 9603: display portion, 9605:     stand, 9607: display portion, 9609: operation key, 9610: remote     controller, 9700: digital photo frame, 9701: housing, 9703: display     portion, 9881: housing, 9882: display portion, 9883: display     portion, 9884: speaker portion, 9885: operation key, 9886: storage     medium inserting portion, 9887: connection terminal, 9888: sensor,     9889: microphone, 9890: LED lamp, 9891: housing, 9893: connector,     9900: slot machine, 9901: housing, and 9903: display portion. 

1. A display device comprising: a scan line and a signal line intersecting with each other over a substrate having an insulating surface; a pixel portion including a pixel electrode and a thin film transistor, wherein the thin film transistor comprises: a gate electrode connected to the scan line; a channel formation region including a first oxide semiconductor layer; a first wiring layer connected to the signal line and the first oxide semiconductor layer; and a second wiring layer connected to the pixel electrode and the first oxide semiconductor layer; a non-linear element formed between the pixel portion and a signal input terminal disposed at a periphery of the substrate, wherein the non-linear element comprises: a gate electrode connected to the scan line or the signal line; a gate insulating layer covering the gate electrode; a pair of first and second wiring layers over the gate insulating layer and overlap the gate electrode, wherein each of the pair of first and second wiring layers includes a conductive layer and a second oxide semiconductor layer over the conductive layer; and a first oxide semiconductor layer in contact with a top surface and a side surface of the second oxide semiconductor layer, and in contact with a side surface of the conductive layer, wherein the first wiring layer or the second wiring layer of the non-linear element is connected to the gate electrode via a third wiring layer.
 2. A display device according to claim 1, wherein the first oxide semiconductor layer has higher oxygen concentration than the second oxide semiconductor layer.
 3. A display device according to claim 1, wherein the first oxide semiconductor layer has lower electrical conductivity than the second oxide semiconductor layer.
 4. A display device according to claim 1, wherein the first oxide semiconductor layer has an amorphous structure and the second oxide semiconductor layer includes a nanocrystal in an amorphous structure.
 5. A display device according to claim 1, wherein the first oxide semiconductor layer is oxygen-excess type and the second oxide semiconductor layer is oxygen-deficiency type.
 6. A display device according to claim 1, wherein the first oxide semiconductor layer and the second oxide semiconductor layer include indium, gallium, and zinc.
 7. A display device according to claim 1, wherein the third wiring layer is formed from the same material as the pixel electrode.
 8. A display device according to claim 1, wherein the display device is incorporated in one selected from the group consisting of a poster, an advertisement, an electronic book, a television device, a digital photo frame, a game machine and a phone.
 9. A display device comprising: a scan line and a signal line intersecting with each other over a substrate having an insulating surface; a pixel portion including a pixel electrode and a thin film transistor, wherein the thin film transistor comprises: a gate electrode connected to the scan line; a channel formation region including a first oxide semiconductor layer; a first wiring layer connected to the signal line and the first oxide semiconductor layer; and a second wiring layer connected to the pixel electrode and the first oxide semiconductor layer; a protective circuit including a non-linear element formed over the substrate and outside the pixel portion, wherein the protective circuit connects the scan line and a common wiring to each other or connects the signal line and a common wiring to each other, and wherein the non-linear element comprises: a gate electrode; a gate insulating layer covering the gate electrode; a pair of first and second wiring layers over the gate insulating layer and overlap the gate electrode, wherein each of the pair of first and second wiring layers includes a conductive layer and a second oxide semiconductor layer over the conductive layer; and a first oxide semiconductor layer in contact with a top surface and a side surface of the second oxide semiconductor layer, and in contact with a side surface of the conductive layer, wherein the first wiring layer or the second wiring layer is connected to the gate electrode of the non-linear element via a third wiring layer.
 10. A display device according to claim 9, wherein the first oxide semiconductor layer has higher oxygen concentration than the second oxide semiconductor layer.
 11. A display device according to claim 9, wherein the first oxide semiconductor layer has lower electrical conductivity than the second oxide semiconductor layer.
 12. A display device according to claim 9, wherein the first oxide semiconductor layer has an amorphous structure and the second oxide semiconductor layer includes a nanocrystal in an amorphous structure.
 13. A display device according to claim 9, wherein the first oxide semiconductor layer is oxygen-excess type and the second oxide semiconductor layer is oxygen-deficiency type.
 14. A display device according to claim 9, wherein the first oxide semiconductor layer and the second oxide semiconductor layer include indium, gallium, and zinc.
 15. A display device according to claim 9, wherein the third wiring layer is formed from the same material as the pixel electrode.
 16. A display device according to claim 9, wherein the display device is incorporated in one selected from the group consisting of a poster, an advertisement, an electronic book, a television device, a digital photo frame, a game machine and a phone. 